I was playing in survival mode after the 1.14 update in a world I created in 1.13 and when night came, I was running around the overworld and I realized that no mobs were spawning. They had the conditions to spawn (dark area, enough space etc.) but there was not anything. Spawners spawn mobs fine, but they are not spawning naturally.

I was in peaceful mode prior to the 1.14 update since I swapped to it to get building done, and after updating from the previous version, and changing my difficulty setting back to normal I played the game to realize no mobs spawn at all.