MobSpawner not checking for air blocks

I heard mobs suffocating when i was using 14w26c, i thought it was lava, but aparently, mobs where spawning on walls and floor, like the spawner wasn't checking for air blocks to spawn mobs, they spawn and died in the walls i guess because as there is a block there, inside is dark?
I build a nice farm taking advantage of this bug, to explain better what happens.