Wrong Aspect Ratio with VR-Stereo enabled

I’m a beginner with Unreal and I’m developing an application using only C++ (no BP) to deploy on Android devices with Google Cardboard.
All works perfectly except I can gain control of the camera’s aspect ratio: I see both left and right images squeezed horizontally and I can’t work out how and where to intervene!!
I tried to play with AspectRatio, bConstrainAspectRatio and FieldOfView parameters of gamecamera reference but nothing: I see no effects of my changes!!

A note: the wrong aspect ratio regards as the launches on devices as VR-preview as Mobile-preview.

Someone else has experimented this issue?

Thanks in advance for any kind of information!