I generated a world in 1.17.1 and quickly updated it to 1.18-pre2 and this happened.
Seed: 2354751386026321940
Coords: -372 74 238
MC-241727
Coverting World from 1.17 to 1.18 cuts Ocean Monument in half.
Resolved
Awaiting Response
0
This one is quite hard to reproduce as the chunks generated depends on your render distance and how much you moved around before converting the world.
If you have your pre-converted world, this would help immensely.