@DustyCowboy 

So there is one thing to know and two things to try.  If you are using only one HS210 you will need to make sure it's connected to the Line or Live side of the 3-way run for it to work properly.  This is because it needs a constant flow of power to ensure the WiFi module is operational.  This allows the voice and app controls to work.   If it is not on the Line side this could be the cause of the concern. 

If it is on the line side though the first thing to do is delete the app and then reinstall it.  Then once installed check for a firmware update for the product (Open Kasa > Tap ME > Tap on Firmware Updates)

  @Carl 

OK, I double checked the connection.
The switch is definitely connected to the Line, not the Load, side.

I also deleted, then reinstalled the app.

The firmware of all the devices is up-to-date.

The Kasa app is version 3.1.5.

Now here is the interesting thing.

After I wrote this (and taking some time to calm down a bit), I decided to give the app one more try.

Of course, now it seems to be working.

There was one instance where it needed to be toggled twice, but I suspect that I may not have waited long enough between tries.

So for now, it appears that things are in order.
Thanks again for your feedback.  Hopefully I won't have to revist this topic again.

  @Carl 

Here's an update:

I installed a second 3-way switch for another light (not on the same light as the first switch so only one switch in each circuit).

I ended up having the same issue as the first one.

Deleting the app, then reinstalling it made everything work.

This was fine for about a day.

Today, the last installed 3-way switch works as expected, but the first installed one now works like it did before the "fix".

It is going to be a real pain if I have to delete and reinstalled the app every other day.

Is there any way to make things more stable???