I have periodic transient disconnection of Chromecast from my Ultra while streaming Qobuz, both directly from the Qobuz Android app and through Roon. The amount of playtime before each disconnection varies greatly.
When Ultra drops the Roon Chromecast connection, the endpoint icon will briefly disappear with the message "Select an audio zone". Then Ultra will reconnect with playback paused. Less often in the middle of a track playback will skip to the next track.
I haven't been able to catch what happens with the Qobuz Android app, but I assume it's a similar transient disconnection sequence.
WiiM Support advised me to use Ultra as a Roon Ready endpoint until the problem is fixed. I'm OK with that if there's high confidence that a fix will be relesased soon, but that's totally unsatisfactory if this drags on much longer. WiiM needs to communicate regularly and openly on this forum about how the issue is being addressed.
My fear is that the fix must come from Google and it's most likely a very low priority if the issue is specific to Ultra. Or worse, in the end no fix is possible.
WiiM continues to sell Ultra as a Chromecase device. We deserve more than being told it's a Google SDK issue, please wait...