The Bug
If you are starting a superflatworld with the present "water world" and looking for an monument, the monument will be over the water level. The water that is around the monument is till y=63 und the surounding water is till y= 42. The monument without the feet is from y=39 to y=60. The feet go down till y=-48. Another problem is that only the 3 Elderguardien are spawned. The normal guardiens are not spawning in or arround the monument.
Reproduce
Superflat: Water World
Seed: 3223233201011443419
Coordinates: /execute in minecraft:overworld run tp @s 78.44 42.72 246.52 -89.85 83.25
Observed Result
The ocean monument generated above the water.
The sea level is at the wrong height due to the world height update.
This setting sets the seabed to sand, but the seabed of the deep ocean biome should be gravel.
How to fix
Update the old settings to the following settings to comply with the 1.18 update:
minecraft:bedrock,64*minecraft:deepslate,5*minecraft:stone,5*minecraft:dirt,5*minecraft:gravel,90*minecraft:water;minecraft:deep_ocean
I am able to confirm this in 21w42a. This ticket clones MC-216015.