I do not know what bugs out this functionality of the addon, but in addition to #4670 there seem to be more situations around the worlds. One more occasion I found is A Great Storm Approaches (12912).

There seems to be a general bug that also causes #4670 but i do not know what causes it.

As if there weren't enough Gremlins, after a character log in /switch/exit, now it's auto-accepting the quest 12912.

Spoke too soon, the exact same quest can't be auto-turned in.

Seems Questie is stuck @ "Checking available quests from gossip" from the debug from what i see.
There is something that bugs the auto-accept/auto-turn in functionality 100%

After a /reload on the same NPC, right clicking gives this:

[12:28:56] Questie: [DEVELOP] [QuestieAuto][EVENT] GOSSIP_SHOW nil
[12:28:56] Questie: [DEVELOP] [QuestieAuto] Greeted by a new NPC
[12:28:56] Questie: [DEVELOP] [QuestieAuto] Checking active quests from gossip
[12:28:56] Questie: [DEVELOP] [QuestieAuto] DONE. Checked all complete quests
[12:28:56] Questie: [DEVELOP] [QuestieAuto] Checking available quests from gossip
[12:28:56] Questie: [DEVELOP] [QuestieAuto] Checking available quest: "The Leaders at Jin'Alai" isTrivial false isRepeatable false index 1
[12:28:56] Questie: [DEVELOP] [QuestieAuto][EVENT] GOSSIP_CLOSED
[12:28:56] Questie: [DEVELOP] [QuestieAuto][EVENT] GOSSIP_CLOSED
[12:28:56] Questie: [DEVELOP] [QuestieAuto] We are done talking to an NPC! Resetting shouldRunAuto
[12:28:56] Questie: [DEVELOP] [QuestieAuto][EVENT] QUEST_DETAIL 0
[12:28:56] Questie: [DEVELOP] [QuestieAuto] INSIDE 0
[12:28:56] Questie: [DEVELOP] [QuestieAuto][EVENT] QUEST_FINISHED
[12:28:56] Quest accepted: The Leaders at Jin'Alai
[12:28:56] Questie: [DEVELOP] QuestieTracker:AQW_Insert
[12:28:56] Questie: [DEVELOP] QuestieTracker.RemoveQuestWatch (by Questie)
[12:28:56] Questie: [DEVELOP] [QuestieAuto][EVENT] QUEST_ACCEPTED
[12:28:56] Questie: [DEVELOP] [Quest Event] QUEST_ACCEPTED 6 12622
[12:28:56] Questie: [DEVELOP] [Quest Event] UNIT_QUEST_LOG_CHANGED player
[12:28:56] Questie: [DEVELOP] [Quest Event] QUEST_LOG_UPDATE
[12:28:56] Questie: [DEVELOP] [Quest Event] QUEST_LOG_UPDATE
[12:28:56] Questie: [DEVELOP] QuestieTracker:Update
[12:28:56] Questie: [DEVELOP] [QuestieQuest] Adding world icon as finisher: 66 70.048899755501 20.904645476773
[12:28:56] Questie: [DEVELOP] QuestieTracker:UpdateFormatting
[12:28:56] Questie: [DEVELOP] [QuestieMap] Unloading quest frames for questid: 12622
[12:28:56] Questie: [DEVELOP] [QuestieTooltips:RemoveQuest] 12622
[12:28:56] Questie: [DEVELOP] [QuestieComms:BroadcastQuestUpdate] Questid 12622
[12:28:56] Questie: [DEVELOP] [QuestieComms:BroadcastQuestUpdate] partyType nil
[12:28:56] Questie: [DEVELOP] [QuestieQuest:PopulateObjectiveNotes] Populating objectives for: 12622
[12:28:56] Questie: [DEVELOP] [QuestieQuest:PopulateObjective] Treasure of Kutube'sa
[12:28:56] Questie: [DEVELOP] [QuestieQuest:PopulateObjective] Treasure of Gawanil
[12:28:56] Questie: [DEVELOP] [QuestieQuest:PopulateObjective] Treasure of Chulo the Mad
[12:28:56] Questie: [DEVELOP] [QuestieQuest:PopulateObjectiveNotes] Adding special objectives
[12:28:56] Questie: [DEVELOP] [QuestieQuest:PopulateObjective] Kill Jin'Alai Trolls until their leaders appear
[12:28:56] Questie: [DEVELOP] QuestieTracker:Update
[12:28:56] Questie: [DEVELOP] [QuestieQuest] Adding world icon as finisher: 66 70.048899755501 20.904645476773
[12:28:56] Questie: [DEVELOP] QuestieTracker:UpdateFormatting

Another one, this time with some debug info.

same thing i noticed (pics and log info in discord channel)
also turn ins.. (certain of them) same gossip nil line

Can you verify if this still happens with 8.4.2+? Entirely possible this was being caused by dialog icon issues with the gossip frame, which should be alleviated now.

I know this can happen because of the API not flagging complete quests as such. Therefore Questie simply can't know a quest is complete even though it is.

I thought Questie uses isComplete to check for that?

True, we changed that recently:

a1ed52e

...but only for Wotlk 🤦🏻