Obelisks of rust in the Red Vale

So my friend I had this bug where we found all three obelisks, got the first rune, then the second one was bugged so we had to close the game, come back to get it, but the third one we cannot get, anybody got an idea how to fix it ?

Got the same problem.

It happens when you die the same time as the boss dies.
The loot despawns, so i cant get the 3rd item also.