I made a custom enchantment for a datapack that would change the mob's movement speed attribute to 0 and store it back later depending on what it was, and while I was testing this on the warden I found some interesting results. While the warden does keep the anger in nbt under anger.suspects, it sometimes also switches to a different target in the array if the movement attribute is set to 0. 

Observed Behavior:
The wardenloses interest or switches target, choosing from the anger.suspects array

Expected Behavior:
The warden will keep anger at the player until it is free to move again.

Demonstration:

Video 1

Video 2

Video 3

EDIT: Read comment for reproduction steps