I've been having a similar issue since 1.16 came out, I've found that switching the realm's selected world and then switching back (which automatically backs up the realm) usually works to auto-save, but you need to do it right after logging out. So far this method has only failed once out of about ten times. Hope this helps while we wait for it to be fixed by Mojang.
OK, thanks!
1.17.1 seems to have fixed the problem.