Philippe von zur Gathen

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, 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...