I am seeing this same issue as well on a 10 user Realm; it seems like the realm server (or process) crashes and reverts to the most recent backup upon recovery. Users will get disconnected if they are playing, but even if they are not they lose any progress after the last backup; sometimes upwards of 20 minutes worth.
Mine forces a re-login to the Microsoft account every time, which requires another device to go to the website. This is a big deal since my son isn't old enough to log in on a laptop or ipad as well as a switch. The only thing I can contribute is that this started happening right around the time of the 1.16.21 upgrade and roughly the same time he started login in on more than one device.
I have observed that when creating a sign, text does appear in the client, but often does not save to the server. If I create a sign I can see the text but other players see only a blank sign. I will see it blank myself when I either log off and log back on, change dimensions, or apply ink to the sign. It seems like the text never saves to the server so any action that would update the sign block with whatever is on the server reveals that it saved as a blank sign.