After converting a 1.17.1 world to a 21w43a world, if a ocean monument is generated in the area where the old and new world generation smooth out, a large gap is generated around it.

In 1.17.1:

• Set render distance to 12 chunks
• Generate a world using seed 1011466919414022920
• Teleport using /tp -30 100 224
  • These coordinates are exactly 14 chunks away from the center of where the ocean monument is going to generate in 21w43a
• Wait until all chunks around the player have been generated

In 21w43a:

• Load the world just generated in 1.17.1 (or download and load 1.17.1 world.zip)
• Teleport using /tp -256 64 224
• A new ocean monument will be generated at the exact area where the old and new world generation merges, leaving behind a large gap in and under the terrain
  

If we generate a new world in 21w43a using the same seed the ocean monument looks like this: