The bug

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

How to reproduce

1. Start Minecraft 1.7.10 (or any version when ocean monuments were not a thing).
2. Create a world with the seed given and teleport to the coords given

   Seed: -1984749456
   /tp -3920 70 10768
   

1. Generate some chunks
2. Quit and save the game, start 1.13.1.
3. Upgrade and load the world.
4. 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.
5. Create the world in 1.13.1 with same seed.
6. 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.