After converting a 1.14.2 world to 1.14.3, previously generated chunks and most newly generated ones after the conversion will not properly spawn mobs, either day or night (easy difficulty at minimum). Some mobs (horses, squids, fish in general, wandering villagers and patrols) seem to be unaffected and spawners function properly as usual.
In 1.14.2 I got the feeling that the amount of hostile mobs at night (even at easy difficulty) were much more than normal.

Screenshots added:
1. Current situation at 1.14.3
2. Same world recreated at 1.14.3 with same seed

3. Backup world opened in 1.14.2