@Jamval I do not have always on enabled. THis is as per the tp-link instructions for using an existing chime in which case always on must not be enabled and the bypass should not be used. I do not need 24/7 recording. 

  @mjbarnard 

I just asked google the following question: "My Tapo D225 is hardwired.  Why am I getting power switching notifications even when hardwired power has not been interrupted?"

Google's Answer:  "The Tapo D225 may be sending FALSE notifications about switching between hardwired and battery power due to a bug that Tapo has acknowledged.  Tapo is aware of the issue and is working on it."

Then I asked googe if Tapo has fixed this bug?

Google's anwwer:  "Yes, it appears that Tapo has fixed the power switching bug in newer firmware versions".

I checked my app and my D225 Firmware is up to date; version 1.1.19 Build 250320 Ref. 42136n.  I have auto update on.

I still get the occasional power switching notifications.. 

But, I just learned that even though my D225 has received the latest update, it may need to be rebooted for the update to take effect.

I'm rebooting now, lets see if that solves the problem.

  @Jamval 

My firmware is up to date. Version 1.1.19 Build 250320 Rel. 42136n

I rebooted the device not through the app but by taking it off the base and pressing the reset button when advised to by support. 

I suspect check the firmware and reboot the device is simply the first line of script that the customer support agents are instructed to read to you on first contact.

However I am currently focussing on whether 10VA is quite enough and planning to swap out the transformer with one that has higher VA. I will keep you posted.

In the meantime it happens without exception every time the bell is pressed. So it is not random, at least for me. 

  @mjbarnard 

  @mjbarnard Yes, my transformer is also a 10 VA which is the minimum requirment for the Tapo D225.  I think I read that you can go up to 40 VA transfomer.  I was thinking of going up to a 30 or 40 VA so I could power a second doorbell at another door.

I don't know if my power switching notifications are triggered by pressing the doorbell button.  I'd have to try it myself, I seldom get anyone comning to the door.

  @Wayne-TP & Jamval

Thanks for your replies. When I measured the voltage it was 28V, although the transformer is set to 24V. Apparently this is not uncommon in UK where mains voltage frequently exceeds the specified input of 230V (can be up to 250V). 

Not sure if that matters but have ordered Vemer (well respected manufacturer) TMC 30/24 which allows 12V-15VA or 24V-30VA. 

I will let you know how that works.

Thanks again. 

  @Wayne-TP I installed the Vemer TMC 30/24 and it works. 

FIrst of all I used 24V 30VA, but the doorbell made a very strange sound. I measured the Voltage and again it was 28V output not 24. It seems the bell doesnt like this (I am using my own chime, not the Tapo supplied plug in one).

Then I wired it for 12V 15VA and it works perfectly - no more changing to battery and back to wired messages. 

Thanks for the advice about trying different transformer. I wonder if using the old one and rewiring it to less than 24V would have done the same - doesnt matter as the transformers are not expensive. 

  @mjbarnard 

  @mjbarnard Unfortunately the problem has returned, despite replacing the transformer. The bell switches to battery and then back again following every press. 

Beginning to think it is a firmware issue.