This should not be happening. One thing I’d certainly try is a simple power cycle of the Sonos Port, to force it to reload the OS.

However, if that doesn’t resolve it, the next time it happens, I would recommend that you submit a system diagnostic within 10 minutes, and call Sonos Support to discuss it. 

There may be information included in the diagnostic that will help Sonos pinpoint the issue and help you find a solution.

When you speak directly to the phone folks, they have tools at their disposal that will allow them to give you advice specific to your Sonos system and network. 

Oh, hold a moment. Is that Denon AVR “Sonos enabled” or whatever phrase marketing uses? It might be that the software on the Denon could be forcing that change. 

It’s an older 1500h denon, so not sure its the sonos enabled but i’ll check.

I can set it to passthrough, but not to variable.