Same issue with the Mu-so Qb 2 update also appeared in my setup today.
I tried everything, changed master clock priority and zone grouping delay, ungrouped the zone and restarted the system, regrouped the zone > the effect lasts about a quarter of an hour, for a few tracks, and then drifts more and more apart. The issue is limited to RAAT. The same problem seems to have been going on at Bluesound for a time without anything being done?
Unfortunately, Raat’s multiroom capabilities were a main factor in my decision to use Roon.