TLDR; Go back to the Maw and make sure YOU have clicked on the named Souls yourselves and not relied on your party doing some or all of the work.
Even if it says 3/3, you will see the named souls in the Quest location you still need to click on. The quest is bugged when in a party and will show as you all sharing the souls freed.
hope that helps 
Hey folks,
I had a problem with this Quest in the Maw today where despite it saying I have 3/3 named souls it wont let me hand it back in. I did raise a ticket but I only got a boiler plate reply, which is fair, it is a busy time.
I decided to have a good old think about the problem, tried the standard stuff like disabling add ons and starting the quest again etc etc.
Then myself and my party I was in wondered if the shared recognition for each soul we had gotten was actually bugged and therefore we just need to actually click on the souls ourselves.
I popped back up to the quest zone, and despite the map no longer showing the quest as active in the area the soul did still appear on my mini map. Clicked on the soul I hadn’t done myself and it completed the requirements to show my quest as done ready for hand in.
I can confirm I was able to hand the quest in too.
I hope this helps anyone else stuck on this.