When drinking a potion, witches are given an attribute modifier that affects generic.movementSpeed with a value of -0.25 and an Operation of 0. As Witches' default value for generic.movementSpeed is 0.25, this effectively sets their generic.movementSpeed to 0. However, if a Witch is summoned with a custom generic.movementSpeed value over 0.25, it will be able to move while drinking a potion. If the custom value is under 0.25, then the witch will be able to move whilst under a slowness effect with amplifier 7 or greater, as 7 gives -105% movement speed. At negative movement speeds, that will become a positive movement speed.

The fix for these strange cases is to just make the attribute modifier for drinking potions be -1 with Operation 2, as this will set the witch's generic.movementSpeed attribute to 0, even if it has drunk a speed potion.

Steps To Reproduce: (from ampolive)

1. Run the command

   /summon witch ~ ~ ~ {Attributes:[{Name:"generic.movement_speed",Base: 1.0d}]}
   

2. Shoot the witch with a Flame bow.
3. Observe how the witch still moves while drinking a Fire Resistance potion.