I was attempting to set up spectator screens to capture some VR gameplay footage from different angles and I am seeing this artifact.
A few details:
*It only occurs in VR mode
*The black bar is actually in the texture generated by the render target
*The width and position of the black bar is determined by a ratio of r.ScreenPercentage and the render target size:
For a 1920x1080 render target r.ScreenPercentage 143 moves the bar fully to the right. As the r.ScreenPercentage gets smaller it moves to the left.
Image of the render target itself captured during play: