When the player is in spectator mode, move doesn't move(they can turn on themselves) and doesn"t spawn.
Linked issues
is duplicated by 6
MC-201343
Rabbits do not pathfind when spawned by players in spectator mode
Resolved
MC-225953
Mobs stop to walk when the player is in spectator mode
Resolved
MC-227635
Most mobs stop moving if there is no non-spectator player nearby
Resolved
MC-236116
Mobs stop pathfinding when in spectator, except for the axolotl
Resolved
MC-257481
Mobs not spawning in spectator mode
Resolved
1 more links
Probably intentional I would guess - mobs only move or spawn if a player is within a certain distance of them - a spectator is supposed to be invisible so that logic isn't kicking in with them.