Seeing as from the looks of Wowhead this bug is 2 years old and it’s not fixed I’m intrigued to see if this has any impact. There’s a distinct lack of response in the “Bug Report” forum by any Blizz staff.
Hey ho - giving it a whack anyway.
Rise, Forsaken - Agatha won’t spawn if you’re doing the quest in Party Sync for the person who has already done the quest previously. Not only that but Agatha when doing the quest the first time is horrendous and painful - having seen in later expansions the companions capabilities it’s rather funny to watch Agatha be this bad.
Either way if Agatha doesn’t spawn - she can’t raise the Forsaken. And not complete the quest.
Which sucks - if people want to play through zones with friends they need to start whole new characters to get through the zone.