I'm having this issue too. Is there a master ticket where this issue is being tracked? There are many other reports describing the same issue.
This is happening for me too on 1.16.1 Java. Reset hours of exploring the new nether, really frustrating. I'll contact customer support as suggested.
We're having this issue too. Once everyone logs off the server it sometimes undoes hours of our work by the next time we log on. When I look at the backups list in the realm settings it seems to sometimes do a backup every 30 minutes or so, but when we experience resets we can see that backups weren't being created regularly while we were playing.
We've been experiencing issues for about 6 days now. If a fix cannot be pushed soon we will request a refund.