It seems there’s some kind of bug with looting.
Specifically, when a mob or boss drops multiple of the same item, the loot window aborts after looting the first one. You have to loot the target again to get the rest.
It’s better demonstrated with an example. I just killed Gruul, and I looted-
Cavedweller’s Climbers
Flare-Eyed Hood
Darklight Necklace
Taner’s Terrible Spine
However, he was still glowing, so I looted him again, and received another:
Flare-Eyed Hood
This is not an isolated example. I’ve noticed this happening frequently over the last few days, with everything from vendor trash to epics from old raids.