Like I said, this has come up before. Took a few minutes on google to track it down, but I posted this about a year ago... 5KFF frame grab with RPP 18mm. The beginning of the vignette is there, but very slight. I have seen others post samples as well and some show more pronounced clipping in the corners, which is weird, but it seems to happen. For all intents and purposes, I would say this lens provides "full coverage", but that's subjective. While it may show some loss in the corners, the 18mm RPP stays pretty sharp and resolves well into those corners.
