1. Load the attached world. Observe that hostile mobs doesn't spawn or spawns extremely slowly.
2. Remove the CavesAndCliffsPreview data pack and reload the world. Note that hostile mobs starts spawning almost immediately.

This test world is created without the data pack loaded (custom superflat world with preset string "minecraft:air;minecraft:plains", structures off). If re-created with the data pack pre-loaded, hostile mobs does spawn (although still at a noticeably lower rate).

Might be related to MC-215444, although reporter of that bug cited mob cap and spawn space as the potential cause, neither of which could be the cause in this case.