I've been noticing ocean monuments won't generate at all or only half the structure generates. The world was created in a snapshot of 1.8 and never downgraded to 1.7.

Whenever I go to a place where an ocean monument should be, only half the monument or only the guardians spawn. This also happens in NEWLY GENERATED chunks as well.

When I recreate the world using the same seed, the monuments do spawn

So yeah, old chunks, new chunks, it doesnt matter, the monuments spawn half (literally half, like the world generates in stripes bug) or not at all.