The bug

Most mobs seem to stop walking (do not wander around / pathfind randomly) when there is only a spectator player nearby. The mobs might start moving, but then eventually stop. They still rotate and can bob up and down, but eventually they never actually leave its position.

Though this is likely intended (MC-46843, MC-48868), it leads to potential gameplay issues as described in MC-202512 and MC-212687. It also behaves inconsistently, because if it is intended behavior, it should apply to all mobs (MC-136206).

Steps to reproduce

This can be reproduced by switching the gamemode from creative to spectator, or by simply remaining in spectator mode and summoning the mobs listed below.

Notes

Logging out and relogging seems to (temporarily) fix the issue for some mobs.

When mobs interact with one another (i.e. cats and rabbits, for example) they might actually move around until the interaction is completed (that is, for example, the cat kills the rabbit). Then the mob stops moving again. Other examples include villagers fleeing from illagers and striders moving around when it rains.

Fixing this issue (if it is not intended) improperly might cause severe performance issues.

Affected mobs

This affects the following mobs: blazes, cats, chickens, cows, creepers, dolphins (MC-202512), donkeys, drowned, elder guardians, endermen (only teleport, don't walk), endermites, evokers, fish (cod, salmon, pufferfish, tropical fish), guardians, horses (including skeleton and zombie horses), husks, illusioners, iron golems, llamas and trader llamas, mooshrooms, mules, ocelots, pandas (they only roll, but don't walk), parrots, pigs, pillagers, polar bears, rabbits, ravagers, sheep, silverfish, spiders and cave spiders, skeletons, snow golems, squids and glow squids (MC-212687), strays, striders, vindicators, wandering traders, witches, wither skeletons, wolves, zombie villagers, zombies and zombified piglins.

This does not affect: axolotls, bats, bees, ender dragons, foxes, ghasts, goats, hoglins, magma cubes, phantoms, piglins, piglin brutes, slimes, turtles, vexes, villagers, withers and zoglins. Shulkers and giants are not affected because they already don't move in creative.

Other mobs, if any, were not tested.

Analysis

Analysis by chumbanotz provided in this comment.