Sadly mob AI got (reluctantly) updated in 1.9 so you can no longer hide in 2 tall plants and behind vines.
https://bugs.mojang.com/browse/MC-3626

Don't throw your tall plants away just yet though, the magical floating sand trick still works!
https://bugs.mojang.com/browse/MC-31038

Its a pain but it happens.
It's a 20% chance to refresh locked trades and enter breeding mode for each trade. Doing multiple trades at once without closing the trading GUI will not give you a better chance for the trade to refresh that doing single trades at a time. It's just bad luck.

Breeding them is not really a solution, because with mob griefing off you have to trade with them to get them to breed.

All hope is not lost though. You could always use a zombie to infect him and cure him again. Note that this will essentially make a new villager with the same colour robe. He will not keep his trades and might have a different career (i.e. Fletcher could come back as a farmer, shepherd or fisherman instead).

If you're annoyed about this, maybe upvote the bug report!
https://bugs.mojang.com/browse/MC-749

This is a great idea. Instead of complaining in-game help bring these bugs to the attention of Mojang.

Also, as a larger vanilla server we may encounter bugs that other people don't see very often (like the guardian spam-spawning) and it would be good for the server if these get some attention

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.

@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.

@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.

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

@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

As pand1024 and I both mentioned in the thread he refers to above, any overworld spawnerless grinder on this server will have a very poor drop rate unless it is incredibly large. Honestly it's something of a lost cause in my opinion, particularly if you want to use it for xp.

If you want a grinder for xp, try a baze spawner or enderman farm. Dual cave spider spawners are also great and fairly easy to find using subtitles.

Regarding the drop distance, watch out for mobs with armour on. Skellies and zombies can have feather falling so they won't die from a 24 block fall.

Did you try it with the horse standing on the rail the minecart is placed on?