This isn't actually a bug, just a questionable design decision. To the left of the button which puts the VC in fullscreen (primarily for watching screenshares) and the button which pops the VC into a window (primarily for watching screenshares) is a volume button which controls the master volume of the VC. That volume button is always there, surprisingly. That it exists at all doesn't make much sense to me, though, considering it is a completely redundant control that duplicates all the functionality of the volume button in the middle control set. The easiest solution is probably to just remove the redundant button.
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Comment by @Koye64