I, and many others, have recently discovered that area spawning is not a thing on plenty of worlds anymore.
My farm world, a brain biome with a diamond mass spawner, not new, has been subject to this bug.
Farming still works on Tiger of Hell, so I suspect maw spawning still works (or it's a world unaffected).
I have also been on a world (deep biome, if it helps) where this does work.
I can safely say this is not just my world, as others have commented that a lot of farm worlds are 'dead'.
I also have first-hand experience as raiding dungeons is peasy now as there are no mobs there either. Or in the environment.@mikelaurence @lisa
Please fix, would be grateful :)