While this may sound pretty cool, it’s not.
This is a intermittent bug. It happen if you are out of range of the spit skill and you ask the best to spit.
The UB will run at the enemy (who is trying to run away: this part is crucial) then if the beast get in range, it will stop moving then not spit or start doing a kinda moon dance.
Sometime the skill will work and some other time even in range the UB will just continue to follow the target.