Objects Missing in Sequencer!

I created a level to having objects moving in sequencer (falling rocks and platforms) about 4 months ago. Upon returning to that level to play test, I have found that all of those objects are missing from sequencer.

They all still exist in the world outliner at their starting locations, but reassigning the objects in sequencer causes the entire track to delete.

I’m not sure what could have caused this or what the solution could be.

Using Unreal 4.18.