Any world generated in 1.12.2 or previous will not keep the structures present when loaded into the new snapshots.

This affects Villages (You have to manually recreate the village by removing and replacing the doors), Ocean Monuments (They will no longer spawn guardians), etc. Newly generated structures in the new snapshot are unaffected.

To reproduce: Generate a world in 1.12.2 or previous. This works best with a version that spawns Ocean Monuments, because it's easiest to see the effects. Find a naturally generated structure that spawns a certain mob, or that you can trace the borders of using plugins (though that would require going non-vanilla).

Load the world in 1.13. Even though the /locate command can find the structure, it won't behave like it should.

Untested in the Nether, that will be my next visit.****

Video demonstrating the bug: https://youtu.be/8HuGCg1W69g