There have  been three occasions I logged in and discovered changes I have made were undone, sometimes with excess of an hour of effort.  I am the owner and only administrator of this Realm.  This appears to be reproducible.

Here are the patterns I noticed.

If I log in by myself and make changes, the changes I made remain when I return

If I log in by myself and make changes, the changes are undone when I return.

If I log in with other people and make changes, the other users disconnect, the changes I continue to make are lost after I disconnect from Realms.  When I connect, my changes are reverted to the moment when the second to last user left the Realm.

There is also an exceptionally long "Connecting to the Realm" time when this happens, it is as if a problem with the world was detected and a backup is being unwrapped into the playing space.

I have experience in software development.  I am willing to participate with experiments, sharing logs, to constrain the conditions where this beg arises.