I did a search on the Mojang Bug Tracker and didn't find anything about it, if we can supply some screenshots and data maybe we could get this submitted as a bug? If it is it would at least get some attention and have a chance to be fixed.

https://bugs.mojang.com/browse/MC-57057?jql=project%20%3D%20MC%20AND%20text%20~%20guardians

Holy Crap.

@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

I am really confused about the entity cap seeing this stats from players. I've been trying really hard to keep mine under 100. I even built my farms, grinders and base hundreds of blocks apart from each other, therefore every time I have to travel/fly for minutes between them, just to keep entities down. So the cap is not 100 anymore?

Entity count is still 100 anyone exceeding it is asked to cull their entities.

Thanks to Baron I submitted this brand new bug report. Be sure to give it some votes so it has a better chance to be looked at!

Voted.
Also it might be worth relating it to this report which probably has the same cause but hasn't had any attention yet.

mmmm. it won't let me vote on the isue despite being logged in.

@FlashFood Thanks to Baron I submitted this brand new bug report. Be sure to give it some votes so it has a better chance to be looked at!

Bug is now marked as resolved for whatever reason, and the mod who marked it as resolved is asking for a forced crash report. Interesting.

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

@NutjobBob I saw that, it sent me an email and I've been debating whether I should find a monument on the server, singleplayer, or even both and do it. I know the environment would change but I highly doubt crashing the server itself for a report is anywhere close to a remote possibility. I'm just glad that its getting attention like that. Thank you for linking that squid spawning "bug," the mod that reviewed the report left that as the only related bug on the list that I had made.

@Shapedsundew9 thank you for the support. Watch it and possibly once I get some kind of crash report up it will become votable again.

@FlashFood Spent 10 minutes on a singleplayer world hovering above a monument and couldn't recreate this - guardians stayed within the hostile mob cap. I wonder if it's something to do with the combination of many players and low render distance.
I did at one point have 123 squid spawn though which demonstrates the other bug.

I've just had an opportunity to play with this myself, do you have any idea what this distance might be? How high is causing them to despawn?

Hey Guys. Didn't realize I made the top 10 list for entities -_-

My bad.

The screenshot from Crafty above of the caves is from under my base. I didn't realize not lighting the underground up would cause any issues. I was quickly reaching 250~ entities and couldn't figure out why. Crafty suggested lighting up everything under our bases. While I don't necessarily like this idea, it may be the only solution until Mojang gets their stuff together.

I dunno if I'm the only one who doesn't light everything up, but this is in part because I'm using a shader so I can just carry a torch in my off hand to see where I'm going. Another reason is to make my area a huge mob grinder/arena and to make it as dangerous as possible. The other reason is to keep x-rayers and cheaters from finding me easily.

I may just try to find a way to trap the areas enough to keep mob counts low. I'll see how it goes. If not, maybe I'll cover the underground in a lot of lava.

So if others aren't lighting things up and are unaware that it is adding to the server lag like I was, then try lighting everything up.

This may not be as big of an issue when simply exploring, but when working around the same area, lighting up the underground areas around it helps. Above ground doesn't seem to be an issue since daylight burns most mobs anyway.

I'm wondering if this is actually a bug in how mob caps and/or spawns are dealt with in multiplayer. Based on a server load distance of 4 the cap for hostile mobs should be about 1,000 with 44 players (not an unreasonable number for the server).

@CraftyMyner If you're finding over 10k entities, that's absurdly high given the mob caps. Even allowing for 100 passive mobs or dropped items per player that's ridiculous. Are you able to post proof of this that could be used in a bug report? It would be useful if entities could be counted individually like in the first screenshots above. Also, is this entity problem something that you've only noticed recently, or has it been happening for a while (i.e. pre 1.9)?

If this bug is unique to multiplayer it might be that this is one of the few vanilla servers with enough people for it to be noticeable. I have still been unable to replicate the guardian spawning bug in SSP.