An actor reference section's default value will not be changed in UMovieSceneActorReferenceSection::OnBindingIDsUpdated

It appears that in OnBindingIDsUpdated the default value is not actually being modified. 

1. Create a new BP actor
2. Add a variable to the BP and change its type to an actor reference
3. In the variable's details enable Instance Editable and Expose to Cinematics
4. Add the actor to your level
5. Create a new level sequence
6. Track the actor from #4 (Custom BP) in the sequence and add another actor to the sequence (e.g. a Cube) as a spawnable 
7. Add a track for the actor reference variable on the custom BP actor
8. Using the drop down menu for the actor reference track select the other added actor (Cube)
9. Right click on that actor's (Cube) binding node and convert it to a possessable

RESULT

The actor reference section's binding is broken. Converting from a possessable to spawnable also breaks the reference.