I solved this in my project. It was using r.ScreenPercentage to set the supersampling. Setting this to anything other than 100 causes the issue.
Using the correct documented commands for Oculus Rift fixes this - vr.oculus.PixelDensity.adaptive on/off and vr.oculus.PixelDensity.0.5…2.0
Note that the r.ScreenPercentage setting persists in the editor so you need to reset this to 100 first.