All UPROPERTYs are serialized regardless of if they’re shown in the editor or not
Not sure what you mean by “created from a C++ that does not include the property”
If you have
UPROPERTY()
TObjectPtr<UMyClass> Component;
and then in the constructor
Component = CreateDefaultSubobject<UMyClass>(TEXT("Component"));
then this object will be created and serialized as expected on this class and all its children