Knil

I think this bug might be tied to maps in the world. You might have to do some testing yourself but on my world if I tried using/updating maps it would let me still play the game but after a while the server would revert as if I never played at all. I only have one other person on my realm so it's easy for us to not use maps but with a larger server I could see this being an issue.

I'm having a similar issue but I want to be sure it's the same conditions you guys are having. I recently updated to 1.13 as well and I went on doing normal things in my world and I noticed after a while I would log back in and my progress would be reverted which I thought was strange. At first I though maybe the server crashed and I didn't notice or maybe I had too much stuff going on in the area but I think I was able to rule those out after realizing that the only time I had a revert was when I was updating the maps on my wall.

What I tested was I would first close the realm, then reopen it making sure it saved my location. Then I would knock the map off the wall and hold it, updating the area. I then would close the realm and open it back up and I would have a revert. So I tried this many times making sure this was my root cause before reporting the bug. See if you guys are getting the same issue too... The area I'm updating had quite a bit of changes done to it but I also updated another map and it also causes reverts too.