Leveling in Durotar. This quest is tied to the timed quest to fetch 4 venom sacs to save a troll in 45 minutes. The quest is left open in case you don't get there in time and need to try again. I was able to complete it, and yet now Questie is stuck with the message "Quests Missing: Finding the Antidote (813)" Can't pick up the quest again, as it is done, and even in my log in Questie.

This is an AAP bug actually (azeroth auto pilot)
You might be able to fix with /aap skip but im not sure

I'm not sure if its a real fix but I just clicked reset questie in the advanced tab and it worked for me