Sequencer does not save asset variables in event struct

When adding a struct to a keyframe, Object Reference variables are not saved and Soft Reference variables are saved sometimes. Opening a keyframe's properties when the selected struct has an object reference freezes the application for a few seconds. In 4.19.2 (CL-4033788) it would crash but that has been fixed in 4.20.1 and 4.21.

This is was reported and tested in 4.20.1 (CL-4233996) and reproduced in Main 4.21 (CL-4234136)

1. Open the UE4 Editor
2. Create a Level Sequence or a Master Sequence
3. Create a Structure with an Object Reference or Soft Object Reference
4. Open the Sequence and add an Event Track
5. Add a keyframe to the Event Track
6. Right-Click the keyframe, Properties > Parameter Struct and select the Structure that was made earlier
7. Select an asset for the Object Reference variable
8. Close the keyframe properties window and reopen it

Results: The asset is not saved in the variable.

Expected: The keyframe should remember the chosen asset.