Nearly march, 2021. STILL Not fixed.
You have to enter again through the tunnel to re-enter the instanced version. If you go through the city (which would seem like a shortcut) you’ll be flagged and stuck in undercity.)
A bug for this long is pretty damned shameful. Legion, BFA, now Shadowlands later, and still no fix in on this absolutely ridiculous quest.
Had the same thing happen after reading everyone else nightmares tried not to fight and hugging the wall. Worked, a work around don’t fight along the green river let the npc’s fight each other. line of sight the npc’s and wait for Thassarian to killed the undead things.
Still happening. However if you can wait until lvl 30 for flying, I’ve found it helpful to just fly in through the sewers, straight to Koltira and not even bother with any of the combat. Takes away from the fun of the quest, but hey, faster and no deaths.
Well, still a thing. Personally it’s not a huge deal to me, but I can’t even revive at the spot I died 
. Oh but it’s okay, I mean, when people ask “Would you recommend WoW?” I’d just tell them that it’s great, if you don’t mind dying multiple times because of poorly thought designs and having to look online for an “answer,” only to find others complaining and solutions that make you do more work than the designers probably intended at first.
But yeaaah, great. 10/10 would complain again.
This quest is still broken… It’s no 2022.
I just used the Draenic Invisibility potion. It worked & finally completed the quest.