Sound problems with 'Mute Audio in Background'

I have sound problems since a while now, maybe someone can help me.
It started already before SU12:

I used to have the setting ‘Mute Audio in Background’ turned to OFF, and it was working fine before.
But now, the sound goes off if I highlight another Window, like e.g. Litte Navmap. But not immediately, after one or two seconds.
That behaviour is so annyoing that I prefer ‘Mute Audio in Background’ set to ON, because then it mutes the sound of the sim immediately when I click on another window.

But even with this setting there is a very strange behaviour.
If I mute my system sound, something I often do, MSFS will override this setting after a while and the sound of Windows in on again.
For example, I usually mute Windows (the setting in the lower right corner) for the 5min startup time of MSFS.
But during the startup process the sim overwrites my Windows setting, and the sound is on again.

Does anyone have an idea what could cause that?
Could it be a Windows setting (I didn’t change anything regarding sound in the last months though), or is that something for the bug section?
I’m using Windows 11.