Multiple times today, I’ve experienced issues with looting both while having AutoLoot enabled and disabled.
With some quest mobs that drop items, for example the Durotar Tiger, I’ve had the loot window pop open, show the item, then close without looting (when AutoLoot is enabled) and I then find that the body is no longer lootable.
I killed over 20 Dreadmaw Crocolisk in search of Kron’s Amulet and over half of them had an empty loot window. ElvUI specifically labelled it as ‘Empty Slot’ - I then disalbed ElvUI and used the classic UI and experienced the same issue - an empty loot frame.