More info!
I had all the track inputs muted in the UR mixer on Mix1. If I unmute just one of the UR inputs the glitching stops. I can repo by muting 7 of them and then muting and unmuting the 8th.
Also if I leave all the inputs un-muted but turn all the faders down to infinity the glitching returns.
Looks like some audio path opening/closing when the inputs are muted? - I dunno, very easy to avoid this issue though.