Strange issue here. There are many threads about unavailable devices but I’ve not seen exactly what is happening here.
I have a various mix of devices that are reporting as unavailable. I first noticed this when I walked into the kitchen and the lights didn’t turn on as usual. In this case, it wasn’t the sensor on battery power that failed, it was the ST outlet that was unavailable.
All these devices can still be switched on/off/dim from ActionTiles but NOT from the SmartThings app. Also, they do not respond to Alexa, she just just says that the device is not responding.