Currently in my world, all mobs have stopped naturally spawning despite being on normal difficulty. Friendly mobs don't spawn during the day. Hostile mobs don't spawn at night. Mob spawners are the only way to generate mobs. I closed the world and opened it and that fixed the issue once but it hasn't worked since that one time. I tried copying the world and faced the same issue. I'd prefer to keep the current world so that friends who join the world will retain their items.

Edit:

I should also add that breeding mobs does still work as intended.