I noticed the same thing: the switch action removed the 0V rather than +V. It can be confusing when the MB-6582 "receives" 0V from somewhere else.
From memory, it would not be so significant if the bridge rectifier was still there.
That’s...weird. I’ll have to check my own MB-6582 to see if I get the same behavior. I only ever use the mix out jack so I’m not sure I would have ever run into this.