Any voice or other controllers set up? 

Make sure the Era 300 doesn’t have a volume limit set.

This can happen if network connectivity is slow. Volume commands are sent to the player and the player replies with confirmation. If the reply is late the controller will assume that the player is not connected and revert to the previously displayed setting.

i can imagine that a nearby microwave oven sometimes interferes with communication. Music is buffered and it can often play through network outages. I have observed half tracks and more continue playing to the end after the network has been shut down. There’s no sense buffering a Volume adjustment response to this degree.

You can get details from SONOS support. Submit a diagnostic immediately after one of these events then contact phone support. Network issues leave tracks in the disgnostics. We don’t have access to the diagnostics here.

Another possibility is that guests were congregating near the player. Humans are basically bags of water and water absorbs WiFi energy. It’s possible that a pile of water bags was stacked near the player as you attempted to adjust Volume. As I noted above the buffered music continued uninterrupted.