So in 5.4 there is currently a bug which prevents you from looting a rare mob although you tagged him.
In my case it was Archiereus of Flame. The mob is bugged for me since a few days, also the last to kills, but i still managed to loot him.
What i did:
1. Fast-Loot was enabled. (It was enabled long time ago, i didnt change anything and im not sure if it works better without Fast-Loot or with it. Mine was enabled tho.)
2. Wait few seconds after the kill, usually there are 10 mobs around you, which can onehit you. Just Wait some secs until the Brawl is gone and the others may have looted
3. What i did: Spam Rightclick (=looting) on the mob 347234923² times. You will maybe get many error messages, but it might still work. WORKED for me 2 out of 2 times i tried.
Small Problem: Problem I had once: If the loot window pops up and you're still clicking, you cancel the current loot window and you maybe won't get the complete loot. Therefore Fast-Loot is the better option i guess, because it loots automatically. First time i tryed this i got Burdon of Eternity, 800 Coints and Epic Item for my class. Second time i only looted one Buff item.
But in the end its better than not looting anything So keep trying !
PS: My theory is not perfect tho, there might be any improvments, so if it doesnt work for you, you maybe click slower or disable Fast-Look!
GL guys!