I tried creating a creeper farm in a creative world as a test. It hadn't worked so I deleted the stuff that made it a creeper farm, and noticed mobs still wouldn't spawn. It is a blank room with a light level of 0.
(Old ticket was on optifine, this is vanilla Minecraft now.)
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I also noticed something with mob spawning:
If you set up a repeating command block with /kill @e[type=!player] turn off mob spawning, and move and load chunks, it says it's killing mobs like sheep even though mob spawning is false
Finnegan O'Donnell, see MC-118347.
Have you tried flying away from the build and returning? Mobs don't naturally spawn closer than 24 blocks to the player.