This just happened to me on PS4. We finally beat aksis after a few hours of attempts and I got the scout rifle off the boss. I then used a Siva cache key on the chest and got nothing, assuming it went to the postmaster. When I got back to the tower, the postmaster was full of blue garbage and green warlock bonds, no cache drop item. So beyond disappointed! We couldn't go to orbit, as we opened all monitors during the run either.