For some reason we are suddenly seeing very few wilderness hostile mob spawns.
We shored up defenses around a village, and added a few homes with beds and doors. The village is prospering greatly, and spawned many additional villagers, more than can fit in beds. 5 iron golems spawned in patrolling.
We also found a zombie spawner withiin the town limits, and created a zombie farm.
We've not seen an illager for 12 days now, and we never once got bad omen even though we killed two banner carriyng illagers.
The village is half on desert, and usually at night its crawling with hostiles... now if we see one or two that is a LOT.
We will see an occasional spider of creeper, but at most maybe 2 or 3 total a night.
We saw something similar on our last beta world, and finally abandoned it because we thought the world was broken...now we are thinking there is something negatively impacting hostile world mob spawns either when you get a larger successful village, or create a zombie farm.
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Something additional I noticed today.. if I'm on alone and get far enough away from the village, I see normal spawning. It could be if more people are playing and one is in a village, and kills spawning worldwide now.
The bigger issue though, is if players are working on a village trading and upgrading it, and the cap kicks in, they won;t see any zombies at all, and more importantly, they'll never see illagers at all.
The point of illagers is to make villages, villagers, and trading valuable, but if players are working on villages, they'll never see the illagers unless they leave and go exploreing.. but then they won't also want to head back to town with bad omen.
Maybe the chances of an illagers spawning near a village need to be increased, or they need to ignore the cap rule to assure the illager/raid/village mechanics come into play?

It's probably the mob cap. There's this other bug with 100+ drowneds spawning in water. Check out any water around for mass quantities of drowneds. Also, different mobs use different mob caps, so the villagers won't count towards it.

Someone suggested chaging the world to creative, launching it, closing it and then replaunching it in survival to clear up spawned mobs.. if I try that however it creates thatt out of space error bug and the game is broken.
Fortunately I made a backup of the world first.

It wouldn't be changing to creative, it would be changing to peaceful that clears the monster caps. That would only affect loaded areas around players.
There is a global cap of 200 mobs, so if your villagers, golems, zombies, and animals are approaching that, it could explain the lack of natural monster spawns.
Illager spawning was fixed in 1.13 so I would guess that's not an issue for you anymore.
Massive accumulation of drowned was partially fixed in 1.13 by a new despawning mechanic for drowned. Drowned can still accumulate, though, when normal zombies drown, due to MCPE-61721, and if they so much as look at you walking by, they will not despawn. That shouldn't affect other surface monster spawns, though, unless you are near that 200 global cap. But check the pools and rivers 64 blocks in every direction from your village.

Thank you for your report!
We're already tracking this issue at MCPE-21856, so I will resolve and link this ticket as a duplicate.
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There's a thing called "mob cap" where the game can spawn a limited number of mobs. The zombies, villagers and iron golems take up most of it.