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.