Willbreaker is not stealing the AI creature.
It only gives it haste and remains on the ai side.
Sounds suspiciously like the other stealing issues reported recently
https://forums.505go.com/discussion/77166/act-of-treason-investigating#latest
https://forums.505go.com/discussion/76992/angraths-captains-command-ability-bugged-post-3-2-update-investigating#latest