Hi there,
I suspect it is not so that the UWP based apps will not work parallel with the Volume mixer or any apps of that sort. This was reported as an update issue with Windows 10 2004 and later rectified by subsequent updates. So how long are you exactly facing this Issue?
I would suggest you to check whether you are in the latest build of Windows.
Here is a thread of the same nature and you can try out the steps preferred by one of the volunteers
https://answers.microsoft.com/en-us/windows/forum/all/volume-mixer-not-saving-settings-for-uwp-apps/0ca57777-196e-4834-b81b-192175f6a065
If the reply is helpful, please Upvote and Accept it as an answer