Changing volume in-game resets volume in Mixer (Windows)

Changing the background volume in-game changes the settings in Windows Mixer. This appears to be an actual bug (Windows 8.1 and Windows 10) and not just something screwed up in my environment. I wonder if it’s because of the file format (OGG).

See it in this video:

That is: if I open up the sound Mixer in Windows and set all volumes to 100, as soon as I change the background audio, the level for that app in Mixer drops to 33ish. Changing the volume in-game makes it change in Windows. It doesn’t make sense.

Longer version: trying to debug a strange bug where the background sound volume affects foreground SFX. Verified that the volume value being passed in is correct (1.0 / not nerfed, multiplied, SoundEffect.MasterVolume is set to 1, etc.) Everything looks fine in-code.

I used git bisect to go back to a previous version of the game that didn’t have background audio. At first, the problem persists - low SFX volume! But when I reset the volume in Mixer, the issue is gone.

I tried this on a second machine with a different version of Windows, and it reproduces more or less the same behaviour. (There’s no “Mixer” in Windows 10, but changing BG audio nerfs the entire game audio.)