This happens during Active Time Battles, 2.16. ORK, 2017.1.2 Unity.
When my combatant gets full timebar together with enemy combatant, on manual mode (not AI controlled), the enemy goes first, as AI picks faster.
However, the Battle Menu pops for my combatant so I'm picking an ability.
Now, when the enemy combatant uses an ability that drains my combatants Timebar, while his Battle Menu already pops, he will not do any of his abilities and will be stuck in "OnTurn" phase, therefore he won't switch to 'AfterTurn' until the whole Battle is over.
All my combatants abilities are set tu 'End Turn' in Ability Settings.
It happens only in manual mode, as during AI controlled picks his abilities already before the Timebar drain happens.