Solved
Old workaround possible for muting
On my Fairphone 4, Android 11, I ran into the known issue with Sound Mode -> Mute that you reported to Google in https://issuetracker.google.com/issues/205205662 (you can add the Fairphone 4 to the list of affected phones). I furthermore noticed that this old workaround on Reddit reliably sets the phone to Silent on my device:
- Set DND Mode "No interruptions"
- (Immediately) set DND Mode "Priority"
- Wait 50 ms
- Set DND Mode "Allow All"
Not sure if it works on the other phones, but I found it worth mentioning.