Yes, we're all in agreement that the mobs are spawning off-screen. But we're trying to work out the details as to which enemies require a certain type of block to spawn on and which will simply spawn based on the biome the character is in. For example, I'm observing that I still get regular cave bats (no ice bats) even when I switch the biome to snow and start getting the snow flinxes.
I do remember making a farm before this update, on the surface, which changed the enemies that spawned simply by changing the blocks. Actually had all that rigged to switches with actuators and it worked like a charm. Don't know if it would work now though.