I just observed a very strange behaviour (Macbook Pro 13 Zoll, 2019, Big Sur Version 11.0.1): when I am using my bluetooth earphones (Jabra REVO a4.0.0), the Touch Bar Volume Slider Widget does not only change the output volume but also shifts the balance to "left" (can only hear on one side).
This however only happens when I have the bluetooth earphones connected, if I select the MacBook Pro Speaker this does not happen, only the output volume ist changed and the balance remains unchanged.
There are many macOS bugs without known workarounds
However the more information the more likely I can find a workaround for the issue. Can you observe this balance change in System Preferences => Sound => Output while you change the volume using the BTT slider?