I expected mobs to spawn (like usual) when I was looking for slimes. I couldn't find any after looking underground for like half an hour, even know I knew where the slime chunks were. But I also noticed that zombies, skeletons and spiders didn't seem to spawn either. Neither did they in the nether. Difficulty is hard (3), and gamerule doMobSpawn is true.

=== Map Details ===
Generator Option: Custom Terrain
Gamemode: 0
Difficulty: 3
Seed: 3714387998929305102
I have custom generator settings as well, though I highly doubt this will affect it.

EDIT:
Mobs do sometimes spawn, but not as common as they usually do. Maybe the passive mob entity count affects it? I have a cow farm with like 50~80 cows in it.

EDIT 2:
Didn't seem to be it, but found something strange: The world seemed to forge peacefull mode, even though the world was locked in hard mode. I couldn't summon in any zombie, skeleton, or any mob that would normally've spawned in Easy mode or higher

EDIT 3: Just found out that mobs can't be spawned in when summoned in spectator mode, but still something strange: Mob spawn rates are extremely low, both in the Overworld and Nether.