follow-up:

I just checked RB 2021, and the master volume slider does the same thing. Maybe somebody asked for this feature so they could raise or lower the volume of everything in their signal chain without changing the mix. I can actually see the value in that. But if I haven't got the mix nailed yet, and hoped to use the master slider to reduce just the backing trax, I need a plan B.

Fortunately, I'm a plan B kind of guy, and this won't stop me. I'll just make sure the master volume is maxed out for every song, then use the track sliders to get the song mix where I need it. I just thought I'd mention this, since I never noticed it before. If its a feature, then great. But if its a bug, you should know its happening.

Speaking as somebody who uses continuous controllers in every song, changing a slider to send a global CC across all channels seems like a disaster waiting to undermine years of work I've put into my sequences. If this is in fact a design.. are there any other buttons or sliders that send global MIDI commands? It would be useful to know about them in advance, and not learn about them in the middle of a song when a parameter changes unexpectedly.