It'll be easy to find the Shellys. Just look for light switches that don't cause the light to turn on or off, or lights that turn on randomly. We put a Shelly 2 on every light switch in our new build 2 years ago (about 30 Shellys) and over 50% of them have already failed.

Usually its either the relay refusing to switch on or off. Or its able to switch on and off via the app, but not via a light switch that it was happily doing the day before. Or it disappears off the WiFi and refuses to connect no matter how many resets / power cycles are done.

After replacing 10 or so of them, I decided it was easier to rip them all out.

Personally I'd prefer if Zigbee just took over everything, then everything would be interoperable and we would avoid being in yet another dystopian hellscape.