The same issue I reported multiple times in the 7.21 beta thread for my RB5009 is still present in the 7.22 beta.
With "Accept Router Advertisements = Yes" in the IPv6 settings, receiving an RA containing prefix info correctly sets a default route, but subsequently receiving an RA without any prefix info removes the default route again, breaking IPv6 connectivity.
As per RFC 4861, RAs without any prefix info should not cause existing default routes from previous RAs to be dropped.
My current workaround is to configure the firewall to block RAs lacking prefix info based on their packet size.
A packet capture and supout files have been provided in ticket SUP-206660.