Migrating a soft reference in a struct from 4.17 to a newer version of the engine results in the variable being changed to an integer

When migrating a struct with a soft reference from 4.17 (or older) to a newer version of the engine, the soft reference will have it's default value converted to an integer after a new member variable is added to the struct.

1. Open a 4.17 project
2. Create a new struct
3. set the variable type to object > soft object reference
4. Save
5. Migrate to a newer version of the engine
6. Open the struct (in the newer version of the engine)
7. Add a new member variable

Result: The soft object references are changed to integers (in the default values panel)
Expected: The soft references would remain soft references