So I've had this occur rather frequenty and I believe I can duplicate it. During the first turn in combat (so far is has always been my soveriegn) with another faction that unit loses their ability bar. Not just spellcasting either. Potions, attacks, and even the wait command are all unavailable.

This starts occuring the first time that auto-combat (not auto-resolve) is used while fighting against another unit. I'm not sure if it has to be against another faction. After this point all faction based combats must have the first turn auto-combat in order to access the ability bar.