Hello there.
I'm currently 4/10 M in Nighthold and progressing Tichondrius, I just recently got my weekly chest upgraded to mythic quality, and managed to get my hands on a lovely 925 Titanforged Ring of the Scoured Clan. Which drops from Gul'Dan on Mythic, the last boss of the instance
I know, I know, I got insanely lucky, and I'll be even more lucky if I down Gul'dan before Tomb releases, which is why I find it utterly ridiculous, no matter how small the chance is, that you're able to get loot from a boss you haven't even killed yet.
Since the chest essentially acts as a free bonus roll, my suggestion is that it only draws from the loot tables of bosses you've already killed. That way you still have a chance of getting an upgrade without disrespecting the raiders that actually killed the boss to get the loot.