Mobs stopped spawning, until in new chunks and skeletons and zombies burn inside caves

I've stayed a few in-game nights AFK, Phantons have been spawning and I've been collecting drops but then I notice no other mob was spawning not even from spawners.

After a while exploring a cave, I believe on newer chunks, I heard skeletons burning and keep finding drops from skeletons and zombies inside caves. Then I managed to find some mobs and saw skeletons and zombie burning inside the cave with no sky access.

I managed to get some clips:

Mob Spawner Not Spawning. Found a spawner and there was burning skeletons inside no other skeleton spawned: [https://youtu.be/c0NUQIsoFFA
]

Mob Burning Inside a cave no sky access: https://youtu.be/QkOuD3VCQPQ