Just updated and now every mob farm on the map is broken, literally wont spawn anything.
Conditions, sky spawners definitely not having the cap taken up tested multiple times with one person in world.
Seem to still spawn on ground level.
Recreate go up to axis y 200 and build a mob farm. The result will be no spawns.
Stood at this normally very active creeper farm for 2 m/c days and nothing has spawned.
Workarounds
Most people should be able to restore their mob farm's operation by placing carpets as the highest block of the roof. If you are currently using bottom slabs or stairs as the highest block, replace them with carpets, otherwise just place carpets on the top blocks.
A workaround for raid farms (mob farms that use pillager outposts) is described in this comment from Jay Sirrom on 24 Dec.
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I did some testing on this bug. What it looks like happened is when Mojang fixed the issue with mobs spawning on blocks they weren't supposed to, they actually instead of only removing the ability for mobs to spawn on those blocks (glass, bottom half slabs, blocks with buttons, signs, etc on them) they actually made it so the mobs wont spawn anywhere at all on that Z axis.
Confirmed in 1.14.1 on Windows 10.
Prowl is correct, but it affects a lot more blocks than just leaves, glass, stairs, and bottom slabs. It looks like most non-solid blocks have the problem, including things like walls, fences, chests, grass path, lanterns, anvil, stonecutters, and many more. My list is up to 34 items so far. If one of these has sky access, nothing will spawn below it at any Y level.
It's easier to list the non-solid blocks that still allow spawns below them:
Carpet
Top slab
Top Snow (1-7 layers)
Redstone Dust
Redstone Comparator
Redstone Repeater
Redstone Torch
Rail (any type)
Scaffolding
Torch
Sea Pickle
Grass
Double Tallgrass
Fern
Impact: This bug eliminates or seriously reduces the spawn rate of:
Anything in biomes with dense foliage, including roofed forests, jungles, most kinds of taiga, and to a lesser degree birch and regular forests. Wolves, foxes, pandas, ocelots, and parrots will be almost nonexistent.
Witches in witch huts.
Guardians in ocean monuments
Monsters in woodland mansions
Monsters in caves under crop farms, most buildings, and any area where the surface has been paved with slabs and stairs or covered with leaves as a spawn-proofing measure.
In a newly created world, there may not be enough animals spawning near the player to keep them fed, forcing them to explore for food before they even have basic shelter. They may not be able to make a bed for several days, forcing them to waste health fighting monsters or to be bored waiting in a hole for the night to pass. It could totally spoil the early game experience and make the game seem much more difficult than it should be.
Yeah I figured that was the case. Good thing I got a good afk session in before the update. On the blocks with buttons, is that only on the top of the block and not the side, prowl? That’s the basis of my mob farm and haven’t had an issue before this.
The Bug. When the highest block at a position is glass, leaves, or a similar spawn-proof block, the Spawner::tick algorithm cancels all surface, cave, and mob spawns on that tick.
The Cause. This bug is caused by the hotfix for MCPE-54241, which was implemented in the wrong location. As a result if the search for a surface spawn position terminates early because it encountered glass, leaves, or a similar block, the game now detects this situation, but will skip the rest of the spawning algorithm.
The Fix. Move the test of the return value of the first call to findNextSpawnBlockUnder after structure spawns are processed and right before the call that will spawn surface mobs. findNextSpawnBlockUnder also has to be updated so it doesn't set the the y-coordinate of the search to -1 whenever it encounters a non-spawnable block when search for a surface spawn spot. Make sure that if the test fails, that only the surface spawn is canceled. Cave spawns should occur even if the search for a suitable surface spot was unsuccessful.
An Example. I've uploaded a test world with 4 pillager outpost spawning areas on grass. When they have sky access, surface, cave, and structure spawns occur. When they are under glass or leaves, all spawns are canceled. Only the highest block matters. When stone and glass above the grass and outpost spawns, you get the expected behavior of cave and structure spawns.
1.14.30 on Android mobile. My witch farm and raid farm are broken. Witches don't spawn in hut, pillagers/captains do not spawn at all. Tried the carpet trick but no change
@Gary the specific bug that this report describes was fixed. There is probably some other reason your structure-based farms are not working. I suggest you get some help with you farms from the community on the Minecraft forums, Reddit, or Discord.
I am experiencing the same problem - my outpost spawns loads of pillagers but never a captain on PE 1.14.3
@Lauren this bug report is about mob farms and structure farms not spawning anything at all. That was due to a bug in which certain roof-top blocks were preventing all spawns below. If you have some pillagers spawning, then this is not your issue.
It is possible that are not getting captain spawns because there is already a captain somewhere within 4 chunks (64 blocks) of the outpost.
We will be tracking the issue of Pillager spawns at outposts but no captain spawns since 1.14 at at MCPE-60560.
Having the same issue.
My once super active mob farm has had no spawns after an 8 hour afk session.