Nether fortresses from pre-1.13 worlds do not spawn wither skeletons or blazes

If you played this world in snapshots, unfortunately fortress is lost: MC-125377

I've had this exact issue. Here is what I did:

1. Create new world in 1.12.2

2. Built a prototype Wither Skeleton farm within the bounding box of a fortress - all mobs spawning as expected.

3. Upgraded the world to 1.13.1

4. No Blazes or Wither Skeletons are spawning on the platform only Pigmen and Enderman seen so far

I've attached the 1.12.2 and 1.13.1 worlds and screenshots. I looked at the Fortress.dat (using NBTExplorer) and it looks the same for both worlds so I assume the fortress is present in both but in 1.13.1 no fortress mobs spawn. You can see these platforms if you enter the nether and go to x=72, z=-299.

thanks, @LogicalGeekBoy.
@Mods: 1.13.1 affected. Also confirmed/community consensus.

Additionally, I have also seen Ghast spawn on this platform in 1.13.1 but according to the wiki "Ghasts will not spawn in nether fortresses." (see https://minecraft.gamepedia.com/Ghast#Spawning)

So it would appear that 1.13.1 does not recognize this area as a fortress after upgrading from 1.12.2. I'm not sure if this affects all fortresses or just a subset.

Logic

You shouldn't use the Gamepedia as a reference, it describes the game including all the bugs.

I can confirm this, the exact same is happening in my multiplayer world.

I worked on a nether fortress farm before the server was updated to 1.13.1 (the world was before on 1.12.2, also created on this version), after the update no wither skeletons and blazes spawn anymore, only loads of zombie pigman and occasionally endermen or magma slimes...

Happened to me too, None of the Fortress' generated while in 1.12 worked.
I traveled far in the nether to generate new chuncks, The Fortress' I generated while in 1.13 worked.
Seems to be the only fix, so I started a new farm there.

Weirdly enough, that's not the case with me. Only the nether fortress that I've built my farm in doesn't spawn blazes and wither skeletons anymore. Another one, which I didn't really touch, only ran around, killed blazes etc still works up to this date. Blazes and wither skeletons still spawn there...

I can confirm this and crying for a fix - I put months of work to build it in survival.

I've been running a server with the same map for 6 years.  About 4 years ago during an update we were told the existing structures' chunks had to be loaded in a certain Minecraft version to save them for the next version.  This broke the fortresses and wither farms back then.  Since then we've loaded in new fortresses and made new wither farms just for them to break again in 1.13.  Can a command in the game be created to where I can specify where structures spawning should be?

I also have this bug. I created my world in 1.12.2, updated to 1.13, then 1.13.1 then 1.13.2, never used any snapshots or pre-releases. So I guess 1.13.2 could be added to the affected versions?

I started building my farm after updating to 1.13. I noticed while building it that no wither skeletons or blazes were spawning anywhere though (in my farm in progress or in the surrounding fortress).

I heard this bug could be related to switching your world to peaceful, though I've only did it twice, once when I was still in 1.12.2 and the second time was after noticing the bug, so I'm not sure if it is accurate or relevant. Thought I'd mention it, though.

Please fix this, I can't get any more beacons and it's annoying. 😞

Thanks.

I can also confirm this from upgrading from 1.12.2 to 1.13.1. The problem seems to be that existing nether fortress bounding boxes have disappeared, causing normal nether mobs to spawn rather than nether fortress ones. For example, I have only seen zombie pigmen and ghasts spawn in my large wither skeleton farm. Note that the ghast spawning mechanic prohibits them from spawning in nether fortresses (1), so this supports my claim of the disappearance of the bounding box.

Newly generated fortresses are unaffected and behave as expected.

1. This mechanic may have changed in 1.13.x in regard to observations I have made.

I am also having this issue. My world was also created in a previous version. Hope there is a fix for this at some point.

Yeah, it happens in 1.14, too (1.14.4 to be more exactly)

It's frustating, in Nether Fortress normal and Wither skellys don't spawn, as well as Magma Cubes and Blazes (yes, they spawn with the spawners), only Zombie Pigman

Besides, I found another Fortress in 1.14 and Wither skellys don't spawn, neither (also normal skellys, Magma Cubes and Blazes)

I built a farm for Wither skellys and since 1.13 doesn´t work. This is a terrible bug, please fix it

(Yeah, I created the world in 1.12.2, I played it in 1.13 and now in 1.14.4)

 
It also happens in 1.15. Only spawn Pigmen and Blazes (on his spawner) in the fortress.

"The problem seems to be that existing nether fortress bounding boxes have disappeared, causing normal nether mobs to spawn rather than nether fortress ones.[..]Newly generated fortresses are unaffected and behave as expected."

yes to fix this you could use a program like mcedit or chunky to delete the chunks you havent build anything on. and save the world. then you could copy paste the content into you save file in DIM-1. thats the content of the nether. if you join again mc reloads the nether. your structures you didnt delete will be there. for instance your base. the mobs will spawn again in the fortress. (for me enderman didnt spawn too. and now they do)

for wither skeletons farms i would recommend structure block to save the structure before deleting. Or using mcedit and copy pasting the structure later again. that should some how work. this way a year ago i relocated my whole base (a castle town) with about 10+ chunks... because of a bug like this when switching to new version my whole world got corrupted. the world looked totally different then shown on  mineatlas for instance. and i wanted to relocate to get the new 1.13 features... so this works 100%. i tested... 

I have downloaded your 1.12 world and upgraded it to 20w12a. I get spawns of all types.