I had the same glitch where she disappeared, just fast travel to New York map and back again worked for me.

Same happened to me first time round on pc I literally reset the manhunt and did it again thankfully it worked the second time.

I was farming for this memento yesterday for hours in the summit and Coney Island as that was where back packs were, had no luck and some guy said to go back to summit and change difficulty from heroic to challenging which I did, then used matchmake joined a group on floor 77 literally did about 10 kills and boom exotic drop memento then low and behold just about to clear the floor and bang another one dropped unbelievable !! so changing difficulty seems to help.