I don't have a quick answer for you, sorry. When you open the mix editor and you set say a few locate points, can you go to them? Does it move in this fashion? There might be something with the time code that is not matching up. Sometimes with stuff like this it is because the time codes are not set the same type or time. SMPTE. BBM, MTC, SPP, (not that anyone uses this any more --- song position pointer) this may be the same as what you are calling BBM.
I know I was getting some drift and a careful check of the Frames per second solved this and now everything syncs up. With my system it saves to flash ROM but I have to check it anyway, just out of habit.
After all these years I have never pulled the trigger on the HUI part of v5.1 so I am not sure that the mix editor and HUI might be separate things. Yea that didn't make much sense, I don't know enough about the HUI function to comment, Wish I was more help, still seems like a setting somewhere if the mix editor is not moving. I personally don't remember having this problem, but I probably did. I have said it before, as simple as it is... MIDI confuses the heck out of me. Just today I had a small issue... tomorrow is a new day. I hope it is a quick fix. Fun though right? An old guy once told me when I was having trouble with work and everything was a drag and nothing was working right and I was just banging my head on everything... just lousy you know? In his wisdom he said... "Make it fun, just make it fun" with a little effort this really does work.
I'll do my best to dig a little deeper on this, but now I got to be up really early to "make it fun" once more.
Michael