For anyone who is still looking for a solution to this problem it solved for me changing the macro GENERATED_BODY() by GENERATED_UCLASS_BODY() then it will recognize the _Implementation and _Validate functions in the .cpp without creating the virtual declarations in the .h.

Also note when you change the macro you will need to add "const FObjectInitializer& ObjectIntializer" parameter to your class constructor ONLY in the cpp file.

Hope it helps!