Strange spawner behaviour

Placing torches around a spider spawner in a dungeon triggered some strange effects:
Skeletons (not spiders) began to spawn only within the bounds of the dungeon, including some within the floor of the dungeon and floating in the air
These skeletons are stuck in the idle animation and stay stationary even when hit (although their animation for taking damage is still present)
Each skeleton spawned can only be hit once; subsequent hits deal no damage and hence they cannot be killed, however hitting the skeleton still produces crit/enchantment particles
The skeletons are spawned rapidly and spawning does not appear to stop
Skeletons disappear but instantly begin to respawn once the save is reloaded

Coordinates:
x = -862 y = 44 z = 873
Seed: 88888888