How to reproduce

1. Create a new superflat world with void preset
2. Make it night time by running this: /time set midnight
3. Fill a layer of stone at layer 63: /fill ~ 63 ~ ~20 63 ~20 stone
   → No hostile mobs are spawning
4. Fill a layer of stone at any other layer: /fill ~ 30 ~ ~20 30 ~20 stone
   → Mobs are spawning correctly on this layer

Note: This bug was introduced in 18w19a