Hello, I am having the exact same problem using the Pimax Crystal on the 5.3 VRTemplateMap VR Preview. I tried disabling Mobile Multiview, but it did not change anything. Also tried disabling instanced stereo with no change.
I found that the large black box in the right eye changes / moves / disappears depending on what your r.ScreenPercentage is set to.
r.ScreenPercentage: 0-50 = no issue with right eye
r.ScreenPercentage: 60 = Very barely start to see it at the top left of the right eye
r.ScreenPercentage: 70 = Starts filling up most of the right eye view
r.ScreenPercentage: 80 = Fills up the entire right eye view
r.ScreenPercentage: 90 = Moves a bit, fills most of right eye view
r.ScreenPercentage: 100 = Fills up most of right eye view
r.ScreenPercentage: 110 = Starts lessening in the right eye
r.ScreenPercentage: 120 = No longer appears in right eye
Here is what the left eye looks like through the lens that works correctly.
Here is what the right eye looks like at r.ScreenPercentage 100
Edit: Further info:
Does not happen if I change it to deferred instead of forward rendering.