The bug
Seagrass on the ocean floor can override parts of the monument.
How to reproduce
Seed: 1325200798562914260
Coordinates: /execute in minecraft:overworld run tp @s -672.50 35.18 -172.80 616.51 22.50
Related issues
is duplicated by
relates to
Attachments
Comments

I think the more likely explanation is that the grass is generating first, then the monument places its pillars all the way down to the topmost block – which can in some cases be the pre-existing sea grass.

Confirmed for 1.13.2-pre2.
Confirmed for 19w13b

Confirmed for 1.16.4.
Can confirm in 20w51a using the provided seed and coordinates.
Can confirm in 21w03a.
Can confirm in 21w05a.
Can confirm in 21w06a.

Can't reproduce in 21w16a. There were changes to monuments in earlier snapshots, so it might be fixed.

Probably fixed in 21w07a along with the fix of MC-211224.

I am unable to reproduce this issue in 1.17.1.

This has indeed been fixed along with MC-211224.