The world was created in one of the previous snapshots

Worlds created in current snapshot don't have the same issue

Re-creating the same world doesn't have the same issue 

At night no mobs spawn in the Overworld.

Mobs do however spawn in the Nether naturally and from spawners in both the Nether and the Overworld

Here is a side-by-side comparison of the the original and a re-created worlds (same location)

2018-11-11_15.22.16.png - original, no mobs

2018-11-11_15.23.54.png - re-created, mobs