FloatCurveAsset reference is lost when reopening a Struct

Structs containing references to instances in the content browser of FloatCurves fail to store these data formats as default values.

This was reported and tested in 4.24.2 (CL-11100242). This was reproduced in 4.23.1 (CL-9631420) and Main 4.26 (CL-11460233)

1. Open the UE4 Editor and create a Blank project
2. Create a Structure and a Curve (CurveFloat)
3. Open the Structure and add a variable for Runtime Float Curve
4. Expand the variable and set the External Curve to the one that was created earlier
5. Save the Struct, close, and reopen it

Results: The FloatCurve reference is lost

Expected: All references are saved when the struct is saved