Defeated Thrale Bonestriker tonight in the Portico, but the quest will not update/recognize the defeat. I cannot progress any further in Celestia with this bug.
I ran that quest last week and had the same problem. defeated Thrale first but as you say, it did not recognized the defeat. Went ahead and fought the other 2 before I or my teammate realized what had happened.
We left the Portico without trying to go any further and re-enterd. But instead of fighting Thrale first, we started with the last one and did them in reverse. We defeated them all including Thrale and were able to continue with the rest of the quest.
It probably is a bug and should be run through Support so that it can be fixed.
But in the meantime, try it again doing the battles in reverse. If you do not get them all recognized then, file a report with support so they can investigate the problem.