Steps to reproduce

1. Replace the floor of a nether fortress with nether wart blocks.
2. Observe the spawns. (Kill or despawn monsters frequently to speed this up.)

Expected result

Magma cubes and skeletons do not spawn (see explanation in original description below).

Observed result

All nether fortress mobs spawn.

In the spawn rules for Magma Cubes and Skeletons there is a spawn on block filter. This would help make Wither Skeleton farms function well (currently Magma Cubes cause issues).

    "conditions": [
      {
        "minecraft:spawns_underground": {
        },        "minecraft:spawns_on_block_prevented_filter": [
          "minecraft:nether_wart_block",
          "minecraft:shroomlight"
        ],

However, Magma Cubes and Skeletons are still spawing on these blocks. Image attached.