Hello
I did a weekly transmog run in the Shadowland Raids today, and I was never rewarded my items in my mailbox from Sylvanas, Halondrus, and Anduin. All 3 of these bosses have their loot distributed from a chest as opposed to the loot of the boss themselves and if you bags are full there appears to be a bug where the loot is not sent to the mailbox from the postmaster.
Despite my bags being full I was still awarded the full loot from the remaining bosses after Anduin.