Mob Spawning is currently broken, but I did some testing in creative and noticed a few things that might help to hopefully further diagnose it:

1) The surface cap appears to be 16. Looks like both cave/surface spawn caps have combined.
2) In creative, the mobs spawned immediately at ground level on a superflat world. Mobs barely spawned in any elevated platform.
3) Mobs seem to spawn at a higher freqeuncy when BELOW the player level. Seems the lower elevation was prioritized vs. the previous spawn mechanics of 25-55 blocks from the player in a sphere. (which was ideal - please revert it back). On surface level you can clearly see the 25 blocks around the player. At any other elevation (even with open sky access) mobs appear to want to spawn directly below the player as a preference.
4) Mobs spawning on single blocks seems to be have been nerfed. Spawning was increased when rows of 2-3 blocks were put together, instead of single lines with a gap/repeat. Even is multiple rows were present with single gaps between them, mobs tending to spawn on solid surfaces more often (in groups of 2-3).
5) Creeper farms are useless with the current cave spawning issues. To target the creeper with a trap door above it is now broken due to the cave spawning rules. There is no way to only target gunpower drops.

This appears to be more than a single cave/surface spawn change. Many more mechanics including the frequency and the priority of mobs spawn appear to have been changed unintentionally.