When Flatten to UVs Space is ON, be sure you are looking the UVs in the right position of the viewport.
Click in the view cube in the Front face.
Another good idea using Flatten to UVs Space, is to set the viewport to "Orthographic" first.
Regards:
Sergio Mengual
3D Artist-3D Generalist-3D Character Artist
https://www.artstation.com/artist/sergiomengual
https://www.facebook.com/Sergio3D2D/
https://www.youtube.com/user/sergiomengual
