Mobs from mob spawners spawn on minecart rails as well as redstone wire.
Steps to Reproduce:
Place mob spawner
Use a zombie spawn egg on it
Surround the floor around the spawner with rails, or redstone wire
Expected: Mobs will not spawn
Observed: Mobs do spawn either on top of the block (if there is not enough room to spawn above it in the air) or will spawn directly on the rail.
Relates to MC-92609
This was always the case, no?