I just cast a spell that defeated Kallah. He disappeared but the battle continued. It was my turn, and there were pips still at Kallah's spot, and they increased each turn. The battle continued without Kallah for two rounds. Then he reappeared with 101 health. I defeated him a second time and finally got the"Victory" message, etc.
There are similar problems with the Guardians in the second battle at Mirror Lake. After the minor guardians were defeated and gone and then the Elephant was defeated, when the Elephant disappeared,the other three reappeared and the battle continued.