this is probably a bug due to my world being created in 1.16, but this terrain being generated in 1.21. i used an ocean explorer map to find the monument, and that map was also created in 1.16.
even the map doesn’t line up! i’m guessing it follows what the terrain would’ve looked like in 1.16?
The outline visible on the map also doesn’t seem to fit reality. Interesting glitch - I wonder how this came into existence - maybe the ocean monument chunks were calculated the moment the map was created but nothing around it was.