I updated my world to 20w21a and an ocean monument that I had found stopped being recognised as an ocean monument. This was a bug for most (if not all) structures at the time and it was fixed in 20w22a for most of those, except that the ocean monument is still not spawning guardians. It still hasn't been fixed by any of the pre-releases or the release candidate.
Ocean Monument at coordinates -401, 289 of seed 7318553948439352260.