OnRenderImage Camera is flipped upside down

I have a script which add an Outline to some GameObject which implements the two methods : Start() and OnRenderImage().

On a simple monoscopic Camera, there’s no problem.
However on a stereo Camera (with HTC Vive), our vision is flipped upside down.

If I put the OnRenderImage() header in comment, the camera is not flipped. In the other case, if there is any OnRenderImage() implementation (even if the method is empty) the camera will be flipped upside down.

Some ideas why is it happening ? and how can I resolve this ?

You need to test whether UNITY_UV_STARTS_AT_TOP on different platforms. See here for further details: Unity - Manual: Writing shaders for different graphics APIs