UnrealHeaderTool and the Blueprint compiler generate inconsistent metadata for object reference variables whose underlying class type includes 'DefaultToInstanced' metadata.
UnrealHeaderTool-generated fields implicitly add 'EditInline' to the variable's metadata so that fields of this type can be instanced directly in the Property Editor at edit time. However, the Blueprint compiler does not currently match this behavior, so variables of this type added through the Blueprint class editor cannot be instanced directly in the same manner.
Define a native "wrapper" BlueprintType struct or class on the C++ side containing a variable of this type, and then add a variable of the native wrapper type on the Blueprint class side.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-194830 in the post.