The bug
Changing the movement speed attribute of the ravager with summon command or the /attribute
command, causes their movement speed to reset to their default movement speed, meaning that you cannot change movement speed attribute of the ravager unless you use a repeating command block.
How to reproduce
Spawn a ravager
Run
/attribute @n[type=minecraft:ravager] minecraft:movement_speed base set 0.0
→ ❌ The ravager still moves
This bug is probably caused because ravagers can "accelerate" unlike other mobs.