@Wayne-TP   First off, all the responders to my original post have said that their transformers meet the minimum output requirements stated in the D225 specifications: 8-24v AC 10VA.  Most US home owners will be using a standard 16v AC 10, 30 or 40 VA transformer.  Conclusion, none of us fall into your category of using an insufficient power supply, in fact most of us are using a transformer that is well above the minimum requirement.  In fact, there isn't a residence in the entire USA that would have an existing doorbell transformer below the minimum 8v AC 10 VA, the majority are 16v AC.  You coudln't even buy a doorbell tranformer below the Tapo minimum.  It's just nonsense to suggest to us that we might be using an insufficient power supply.  This is why Tapo designed the D225 for that range of transformer, virtually all existing and new doorbell transformers will meet the requirment.

Second, all of us have checked the output of our tranformers, and all of us have verified that they are outputting the voltage stated by the tranformer's manufacturer which in all cases is well above the Tapo minimum requirement.  None of us have reported using a faulty transformer.  Some responders have indicated they are using a new transformer.

Third, if you know anything about simple step down transformers (that is what we are using) they are either working or they are not.  It is silly to ask us to check output voltage at the random time when we get the random power switching notification.  The most common issue would be if another device is also connected in series to the same transformer and is randomly drawing output below the minimum.  None of us, including myself have any other device connected in series to our transformers.  Existing doorbell chimes are always wired in parallel, for that exact reason, so there is no voltage drop.   Under what electrical circumstance would a simple electrical step down transformer randomly output a voltage drop from 16v AC to less than the minimum 8v AC with no other load connected in series to the circuit?  Most possibilities would cause a constant voltage drop below the minimum, not a random drop.  One possibility is poor wire connections at the tranformer or the Tapo D225, but all responders have indicated they double checked their connections. Unless the poor connection is a cold solder joint internal to the D225, which is Tapo's fault. Another possibility is flucuations on the primary side of the transformer, the input 120v AC.  If there was enough fluctuation on the primary 120v side to cause such a proportional output drop from 16v to below 8v the responders would be experiencing far greater problems throughout their homes.  Another possibility is the connected device (Tapo D225) creates a poor power factor where current and voltage are not in phase which causes the transformer to have power drops.  This would be a Tapo design fault.  

This is typical of electronic device manufacturers, point the finger at another component in the system.  This is why I bought Tapo devices under the TP-Link parent, because I have a TP-Link router, if something goes wrong you can't blame the router manufacturer.  But, now we see Tapo/TP-Link trying to blame the poroblem on the transformer.  

It is not my tranformer!  I had a ring doorbell V2 that was working perfectly for several years hard wired off the same transformer.  I got rid of the ring because I was tired of having to pay a subscription to see event videos. 

When I googled the power switching notifications for hardwired Tapo D225 the AI generated response says that there is likely a bug in the D225 power management and that Tapo has acknowledged this bug and is working on fixing it.  

I guess when all else fails, blame someone else.

  @Jamval Interesting points and I tend to agree.

I wonder if the scenario you and I are both identically experiencing is relatively unusual and so Tapo are not aware of it and perhaps it isnt even being flagged where it does occur.

If I read your original post correctly your setup is same as mine - using original chime, not using jumper and not selecting always on.

How many Tapo are hard wired? Might not be the majority.

How many Tapo hard wired use the supplied chime. Probably most. How many use an existing chime with always on not selected and not using jumper? Might be relatively few.

Then of those few, how many would bother to register this? Perhaps vanishingly few.

At the end of the day, my doorbell rings, it records and it switches straight back to powered a second or two after reporting switched to battery when it is pressed. So it works. I only searched and then posted because I feel innately uncomfortable when an item does not appear to be working as intended. 

This issue in this particular scenario/setup might be more widespread than appears but most dont know, understand or wish to report. Might be worth checking, as my Tapo messages of switching between battery and powered are identical to yours, only difference is I have identified it occursevery time the bell is pressed. Could probably upload a video demonstrating.

Slightly annoyed that even after purchasing a new transformer (around US 50) which has options for 12V and 24V and for 15VA and 30VA still does not appear to function as intended on any of the (compliant) settings. 

Thanks for your persistence in attempting to understand this. FWIW I changed from Nest (google) doorbell due to such excellent experience with TP-link ethernet switches, whole house Deco mesh, several Tapo smart plugs and three other internal Tapo cameras. So I voluntarily changed, and I have to comment that the responsiveness of the cameras and notifications is far superior to Nest.

  @mjbarnard There are 3 components in your circuit, the transformer (power supply) and 2 loads, the Tapo D225 and the existing doorbell chime. I think we can eliminate the transformer as the source of your porblem since you have tried 2 transformers.  We also know that you get the power switching notifications when the doorbell button is depressed.  We also know that you have your existing doorbell chime active to chime when the D225 button is depressed, correct?  To me this indicates two possibilities. The Chime or the Tapo D225.

Your existing doorbell chime could cause enough voltage drop to cause the D225 to switch over to battery power and then back to hardwire when the doorbell button is no longer depressed.  Within this possibility there are two conditions which could cause this.  (a) the existing doorbell chime is wired in series with the Tapo D225 and your transformer.  Loads in series will cause a voltage drop.  This is not likely as I've never heard of even an old doorbell circuit having the chime wired in series, they are normallly wired in parallel just for the reason of not causing a voltage drop.  (b) your existing chime is wired in parallel but is faulty causing a voltage drop.  In either case the easiest way to eliminate the chime as the source of the problem is to bypass the chime, refer to the D225 instructions sheet on how to bypass the existing chime.  Then depress the Tapo D225 button and see if you get the power switch notification when the chime is out of circuit.  If the D225 does not report a power switch then you know that your existing chime is the source of the problem. 

If you still get the power switching notification when the chime is bypassed then the source is most likely the Tapo D225.  Depressing the doorbell button could cuase enough physical pressure on internal circuitry and connections to interrupt the hardwired power supply, which triggers the power to switch to battery.  Double check the 2 connection points of the existing doorbell wires to the back of the Tapo D225.  If that does not solve the problem there could be a bad solder connection on the internal PCB's of the Tapo D225.  This would exhaust my trouble shooting to the point that I would be satisfied concluding that the problem is with the Tapo D225, and if it is still under warranty, I would demand a replacement.  

Thanks that is super helpful. Very grateful for the information/education.

I was certainly concerned about pressure on the bell disrupting the connections. So I triple checked the connections points and in fact reconnected them very carefully to exclude this as the cause. 

 Also it is a new build modern house, around 10 years old so I would be surprised if the wiring was not parallel. 

  @mjbarnard I just depressed the D225 doorbell button.  I did not get any power switching notifications.  In fact, since I checked for firmware updates and rebooted the D225 a week ago, I have not received any power switching notifications.  One of the google AI responses I got when looking for a solution was that the power switching 'bug' would have been corrected in the latest firmware version.  Also, for a firmware version to take effect, the device must be rebooted from within the Tapo app.  Apparently just downloading the latest firmware is not enough, you must also reboot.  Go to device settings for the D225, scroll down to where it shows the firmware update version, just below that is "Reboot Doorbell" in red text.