I logged in at around 6 local time and started playing. I got off and on once or twice. Around ten I logged back in and the world was in the state it was in when I started. All items gained since then were lost and the structure I was working on reverted. The owner of the realm experienced the same thing.

The only thing I can think to add is that in the previous instances where I logged off and on again the owner was also on the server. The bug occurred when I logged back into an empty server, perhaps it was overwriting the current state with a backup?