Two times, two different players, two different parties with different players (I was in the group the first time) in their groups in my discord encountered a bug where they defeated asteroth and when they went to the next questline event, it would state that the whole party needs to be present. They would leave group reform the group, then have to fight asteroth again. That is all.
Try playing solo 
This game was built for solo play first. Also, lucky you got to run the fight 2x.
This is a “bug report” forum fella. Not a “Jump in the conversation uninvited and tell them their bug is not relevant” forum guy. Go troll somewhere else junior.
You assume it is a bug
You got the downs or something kid? Go bother someone else champ…
You have been flagged, we do not tolerate your “style” of talking here.
That’s hilarious. Now you are trying to pretend you work for Blizz… Wow. Just wow. Also you are ignored and I have the last word. I bet that infuriates you. 
It happened to me too. I was playing solo. Beat him, logged off for a few hours, logged back in, now the story reset to before the battle.