On the snapshots of the update, previously generated Nether Fortresses are not being recognized as actual nether structures anymore, meaning that blazes and wither skeletons don't spawn. I had already explored a lot of the nether near my portal so newly generated terrain is ~1000 blocks away in any given direction. Because of this, the /locate command says that the nearest fortress are in the newly generated chunks, and not in the ones which previously were recognized by the game as Fortresses.

This has ruined my platforms which I use for wither skeleton killing, as I would have to travel 1000 blocks for them to spawn. I don't know if this is a snapshot-wide issue or just a world-specific one.