Chunks Reseeded After Overnight Backup?

Yes, I did look for similar tickets, but this bug appears different enough to be worthy of a new ticket. Also, this is not PE.

I am running a small Realms server (never more than 8 people on at any given time, probably a total of 16 people on the player list, 25% of the inactive).

Last night, I used /locate to find the nearest Mansion, and then I and another player started working on a build at this location (over 8000 blocks from the original spawn point). When I logged off for the night, I was still at this location.

When I logged on this afternoon, I spawned closed to the original world spawn point (which we had made a small custom build at). Over 50% of this build and the terrain was COMPLETELY altered or gone entirely. Originally, this was 2 islands in the middle of a vast ocean. Suddenly it was an island and then a jarring wall of new seed of beech forest right next to it. My inventory was also completely cleared. I picked up a random piece of something and got an achievement for something I had done ages ago. It was like a complete reset.

I went to look at the last saves. There was one from 12 hours ago, and another from 13 hours ago. Hoping I could just load the previous save and everything would go back to normal, I did.

Before I loaded this save, I had logged off at the original spawn point (now half there). When I loaded this save, first I got some weird error message, like the load had failed. But this happened to me before and it still loaded fine, so I ignore the message and loaded up the Realm. I arrived in an entirely new world seed. The terrain was wholly different. None of the build was there any more. None of the command blocks I had put down. I still didn't have anything in my inventory from my log off from last night.

I was hoping not to lose any progress in our build. But at this point I was desperate to get anything back. When I went to go find the "13 hours ago" save, the World Save log had 15 new logs, all seconds apart. The "13 hours ago" save was still there, so I tried that one.

Again, same weird error (500, I think?) and it created 15 new saves seconds apart, BUT this time it logged me in where I had logged in last night, and with all the things that were in my inventory.

Everything appears to be back to normal, but this is this first time this has ever happened to me. Would love to know that this won't happen again. Got lucky and didn't lose much progress this time, but don't look forward to having to rebuild something should this happen in-between saves.