No, I did not submit a support ticket. Apparently, the beta & RC threads are supposed to be read by some MikroTik staffs, and we are supposed to raise any issues encountered with the version being developed in those threads. But for this case, they probably won't consider changing the introduced behavior.
In that thread I also suggested the real fix for the problem (bad preferred prefix after reboot) V7.21rc [testing] is released! - #41 by CGGXANNX, that even got 12 "thumb-ups", but MikroTik seem not interested by that idea either.
The problem is only bad for Windows clients (I don't know about Apple devices though), Linux and Android, although listing multiple IPv6 addresses as preferred, will pick the more recent address for new connections and has no connectivity issue (unlike Windows).