Structures regenerate, replacing existing blocks, if structure generation is enabled

[Pokechu22]

Structures can regenerate (at least with ocean monuments) where they are already present, which will overwrite player-placed blocks. This can cause major world damage.

To reproduce:

1. Create a server with the world seed set to 4830895116345664966 and the generator set to default.
2. Join that server. Teleport to 192 64 256 and build a pillar on top of the ocean monument in this area, as pictured:
   
3. Start downloading.
4. Run /seed to set the world seed and generator.
5. Enable structure generation in generator overrides.
6. Stop downloading.
7. Observe that the saved singleplayer world looks like this:
   

This is probably caused by structure data files not being generated by WDL. Unfortunately generating these files is rather difficult, so an ideal fix for this won't happen any time soon - instead consider leaving structure generation disabled. Alternatively, you can download the world, run around to generage the structure files (and have the existing world get damaged), and then download the world again.

Reported by Roshy76 on the forums.