That's super odd. Normally I would be pointing to the uninstall/reinstall procedure (with all the reboots and such) but honestly you strike me as having tried all the basics already.
I hope Vincent has some idea?
Would you mind explaining why? Generally, you'd set it appropriately (usually this means leave it at 100%) and adjust volume in voicemeeter. I can give you a better answer if I understand the use caseI often get to go through this sequence