Same thing here, reported twice in the past (and tested FEW times by my guildmates). Either something wrong with NPC itself (well, tried to kill it and make it "force-reset" by respawning - ...
WoW Freakz link: https://www.wow-freakz.com/quest_helper.php?quest=9176 (https://www.wowhead.com/quest=9176/the-twin-ziggurats)
Bug description: Unable to complete, can't turn in (despise having both ...