dang, how did you pinpoint that crafty? is it random flying or some kind of map u can look at?

@FlashFood I did a search on the Mojang Bug Tracker and didn't find anything about it

This is known about (it's mentioned briefly in the wiki ), but still seems to be a bit obscure and for some reason hasn't been submitted as a bug. Would be a good idea to submit it with screenshots.

As far as I know this happens because of the order of events within each spawning cycle. The game only checks the mob cap once at the beginning of the spawning cycle, then if they're not exceeded picks a bunch of random locations around the players to try to spawn mobs. Normally most of the spaces are invalid, and everything is fine.
However for guardians and squid there can be a huge number of valid spawning spaces (a monument has up to 77,000 possible spawning spaces for guardians). Meaning the game can spawn hundreds in a single game tick, massively exceeding the mob caps.

The problem then is getting them to despawn if no staff is online to help. Leaving the chunks won't do it because they'll be unloaded before they despawn. Waiting is an option, but they will take time to despawn. The best method is probably to pillar/ladder upward to y>200. I would suggest anyone spending lots of time at an ocean monument install a giant ladder :P

Thanks crafty for trying to help me with the random disapearnce of my room.