I pulled the bearing off the end of the brush roller (it does pop-off fairly easily) and found a lot of fibers and some sticky oil. I cleaned all that off and added a touch of good lube and it has improved it, also quieter. It ran most of the time on the Berber (including on turns) and stopped less often on the medium loop.
I noticed it will run continuously in one direction and stop a couple times in the other (carpet grain difference).
So it is the amount of friction, and maybe the firmware being too picky since surely I shouldn’t be needing to do this like 1 week in already.