See title!
Obviously this is a funny little bug, because I’m soloing. For context, I was playing assassination rogue, killed the Maiden first, then swapped and must’ve crit, because the floor stayed whole and Avatar fell. lol Oh well, not a big deal, but maybe add a teleport to the bottom at the elevator or something.
Just happened to me as well on my assassination rogue. Seems insta-killing him with Sudden Demise talent might be causing it. Not sure though, couldn’t figure a way to get my loot either. Maybe it will be mailed to me.
Wish they’d add a teleporter or button to press to force progression cause this just happened to me
In case anyone else runs into this - I ran back to the elevator, took the portal back up to maiden then jumped down to kill myself. Releasing put me down by the fallen avatar corpse where I could loot and then continue my run.
I recall having this happen at least once in Tomb of Sargeras, late in the life-cycle of Legion, when everyone was broken OP from full sets of raid gear, double legendaries and excessively juiced artifact weapons. And, just like you, the only solution we could work out was to orchestrate our own deaths to advance the raid progress.
Tried this option - it did work, but Khadgar didnt spawn down there so I wasnt able to use the portal to Kil’Jaeden. RIP 
Still a problem as of 11.1.0. I had the same thing happen, except on Heroic and not Mythic. Don’t see this being fixed in 11.1.5, but maybe I’ll be surprised.