The Bug

Spawners with mobs like blazes, which are immune to fire, don't spawn in lava.

How to Reproduce

1. Prepare a lava lake:

   /fill 14 -61 14 24 -58 24 stone
   /fill 15 -60 15 23 -58 23 lava
   

2. Place a mob spawner above:

   /setblock 19 -59 19 spawner{SpawnData:{custom_spawn_rules:{},entity:{id:"minecraft:blaze"}}}
   

Note

It doesn't affect striders or magma cubes.