Saw this issue previously. A similar issue was fixed with Bows and since I wasn't certain if my previous instances of it involved bows I figured that was the issue. However this time i'm certain there were no bows involved.

To sum up Leader of AI party dies. It won't end their turn. Your in limbo, only recourse is to auto resolve or alt+tab and end game.. reload and hope it doesn't pop up again.

As you can see in the pic below the Listed AI leader is dead, Since you can't save game in middle of tactical combat screen shots is best I can do.

I ended up Auto-resolving but if it were a closer battle this would/could really bother people. Not even sure how we can help with tracking down this issue.

He died on his turn, attacked me and was killed in a counter attack. Once he was dead the other unit went, then I was stuck in turn limbo. I could speculate as to the root of the issue but my speculations are usually way off base..