Sounds like you may be seeing an issue when the Arc goes into sleep mode when not in use.
I’d call Sonos when you can be by the Arc, and get them to look into the issue.
Seems odd, I’ve never had a “sleeping” Arc leave my network at all. They’ve always shown up when I look at my controller app.
I’d have thought it might be more likely some sort of network issue, where the Arc is losing connection, potentially due to a duplicate IP address, or possibly some other sort of wifi interference.
Would certainly agree that submitting a system diagnostic within 10 minutes of experiencing this problem, and calling Sonos Support to discuss it would likely help.
Did you resolve this? I am having the same problem. Sonos Arc works fine but then drops off the app once it goes idle. I am unable to reconnect it without powering off completely (from the mains).
I’ve reset the Arc, deleted & downloaded the app again & nothing works.
When in sleep mode the Arc is completely unresponsive.