There are a few comments on Wowhead finding problems with this quest.
In Cataclysm, when the devs remade the quests there, they got kinda fancy with special effects and triggers, and it didn’t work out so well. Mostly, the quests work. Mostly.
The Alliance quest They Took Our Gnomes, made as part of the same set, is infamous, and I have seen reports of, and experienced, other fails.
See also https://us.forums.blizzard.com/en/wow/t/quest-sabotage-in-southern-barrens-bug/1902696 from the US Bug Report forum
I have known other quests in that area to clear up after a weekly reset (Wednesday morning). Apparently the server can get stuck on something after someone completes the quest, and then some trigger event necessary for the quest can’t happen until the server is restarted.
So: move on and try again Wednesday is one solution.
Or you can just move on and don’t worry about it, since you don’t actually need it for anything. Unsatisfying, but practical.
Or you can make a Support Ticket. A GM will likely just mark it completed for you, since they can’t interfere with the server at all. but your wait times may be very long, since this is the first week of a new expansion and bugs and glitches are everywhere, and I imagine all types of support personnel are way overloaded. Also unsatisfying.
Personally, I’d move on, not worry too much, and check again after weekly reset.