First time that happened and I don’t know how to reproduce it.
We were playing Athel Yenlui and got a Stormfiend after the first drop in the chasm before it splits into to ways. We killed it (unfortunately I can’t remember how) but the quest did not complete:
In the Clan Fester Camp a Rat Ogre spawned for us, we killed it and the quest completed:
I’ve attached the console log. Looking through it I noticed the timestamps of the screenshots and the log entries differ by one hour. So although the first screenshot was taken at 17:40:14 the corresponding part in the log happens around 16:40.
console-2021-01-12-15.34.32-20dd0141-392d-4279-b547-fade9c2fb81d.log (3.1 MB)