Did not receive any loot, including quest item on one character after completing the Mega Dungeon “Dawn of the Infinites” for the first time on that character. I have completed the dungeon multiple times and never experienced this issue. I received loot from every other boss except this one. Waited to see if anything would show up in my mail before posting this / making a ticket.
Dragonflight isn’t considered legacy content yet so you aren’t guaranteed any loot at all. The fact that you got loot from all the other bosses is honestly the way more surprising thing.
Is this the quest item you were talking about?
Looking at WoWhead it shows it as being roughly a 31% drop chance. That could be wrong, I’m not sure, but that would explain why you didn’t get a quest drop as well.