im having the same problem
the worst thing is that in some weird way it does count as done and you are unable to reenter the actual story instance again (only allows you to reenter to complete achievements) even though it still shows it as uncompleted in the story journal and if you were to cancel it you would always be stuck with that annoying gray star indicating unfinished story
so the only way to get another try at actually completing the story is to completly restart the story
when i went in to retry the last instance after the bug happened to me the first time i didnt realize the retry was just a replay version for achievements instead of the real isntance that would count for the story but during that try it worked flawlessly... i then restarted the entire story to try again to actually finish it and ofc it bugged again and i rly dont want to play through the whole story again just for another single try at the every end for it to maybe work