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.

A Minecraft screenshot of a player holding an ocean explorer map. It is barely explored, however we can see that there is a big blue square surrounding the monument, and that the outline of the unexplored terrain does not match the outline of the explored terrain.

even the map doesn’t line up! i’m guessing it follows what the terrain would’ve looked like in 1.16?