It's mostly shifting right and it's an issue that existed forever. The reason for it is that the microphones were positioned on the left side, and OS X thinks it's smart enough to move the balance to the right, so that music/audio doesn't interfere with microphones (might be helpful during calls). Skype used to do the same.
I am using a small app called Balance Lock https://apps.apple.com/us/app/balance-lock/id1019371109?mt=1... to automatically restore the balance. No affiliate with the author of the app in any way.
