Note: the below happened on Juli 26 on the character I am posting with.
I tried looting the Blade of the Prime from the object in the cave in the Dread Wastes however no item got in my bags, the loot window opened and closed when I got hit by the named mob that spawned just as I had the loot window open.
After killing the mob, the lootable object was no longer there. Nor was it lootable again in the days after (I checked several times a day for over a week now, on a mostly dead realm.
Turns out, on opening the object, your character is flagged with an invisible quest. This then makes the object itself invisible and thus no longer lootable by the character even if it did not actually get the item.
The quest in question is: 31433
Checking with the following command on that character yields ‘true’:
/dump C_QuestLog.IsQuestFlaggedCompleted(31433)
Even though I never actually looted the item on that character.
I opened a ticket, but after some back and forth got directed to the US bug report site (???) so instead I am just opening the ticket here.
This same issue was also noted on a wowhead comment from years ago:
Is there a way I can get this hidden quest complete removed from this specific character so I can attempt to loot it again or is it game over?
As reference, my original ticket was [EU100426258]