I managed to lock up Zixzax.
I entered the Hall of Heroes and spoke to the Mysterious Spirit.
I then dismissed Madora and spoke to [Henchmen trader].
When I left the Hall of Heroes, Zixzax acts as if I never went in. Entered and re-entered the hall and still the same. Same after adding Madora back in.
He also locks up if you Rift Travel out of the Hall of Heroes. When you run back up to him at the End of Time he acts as if you never entered the Hall of Heroes. Entering and exiting the Hall of Heroes works to fix it in that instance though.