When you switch from creative or survival mode to spectator mode, some mods such as squids (passive) or zombies, skeletons, spiders (hostile) despawn.
Linked issues
duplicates 1
Comments 6
In MC-46776 :
@unknown:
I've changed it so that mobs will despawn even if everybody is a spectator, to be consistent.
Works as intended. Duplicate of MC-46776