When you try to upgrade a world from 1.7.10 to 1.13.1 and the chunk has already been generated in 1.7.10 where the ocean monument should generate, it will not generate a ocean monument but will still 'say' there is one if you use /locate Monument.
This does not happen when upgrading from 1.7.10 to 1.8 the monument will generate, See upgrading from 1.7.10 to 1.8.png
- Start Minecraft 1.7.10 (or any version when ocean monuments were not a thing).
- Create a world with the seed given and teleport to the coords given
- Generate some chunks
- Quit and save the game, start 1.13.1.
- Upgrade and load the world.
- Observe how the ocean monument didn't generate, but if you type /locate Monument it will still give you coords to the monument that didn't generate.
- Create the world in 1.13.1 with same seed.
- Teleport to same coords and there will be a ocean monument, See when created in 1.13.1, same coords.png
Relates/can relate to MC-126161.