Edit: The bug is that the world seed was spontaneously changed either by loading or exiting the server client. Perhaps it was caused during a server crash. Based on two other occurrences I found of the same event, I can conclude that the bug has been present through many versions and happens only when a singleplayer world is hosted as multiplayer using the server.jar executable.

https://www.reddit.com/r/Minecraft/comments/4ypzhb/worlds_seed_randomly_changed/

https://www.minecraftforum.net/forums/support/java-edition-support/2753497-help-my-minecraft-world-seed-changed

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Original message:

• Treasure maps' unrevealed terrain does not match revealed terrain.
• New chunks do not match old chunks-"terrain scar".
• Spawn point moved to a new location.

I didn't have this problem initially when playing 1.13.1, 1.13.1 pre-1, or 1.13.1 pre-10. In all three versions I spawned new chunks and used treasure maps without issue, and in one of them I respawned at the world spawn which was still in its normal position.

But today (in version 1.13.1 pre-1), all new chunks do not match old chunks, all treasure maps do not match the terrain, and when I respawned at world spawn, it was in a new place. My version has not changed recently. I am using the server.jar of 1.13.1 pre-1 but I had spawned new chunks before in this very server version and did not experience terrain generation issues or a terrain scar.

In the image, you see the player in chunks originally spawned a few days ago in version 1.13.1 pre-10 server (IIRC), and ahead you can see new chunks spawned today in version 1.13.1 pre-1 server. After encountering this bug, I changed to 1.13.1 server to see if it was still happening, and it was indeed. New chunks generated match the terrain in the distance in the image, and do not match the foreground terrain.

This world was initially created in one of the 1.9 snapshots months prior to its release.