Seems you did not understood the whole point.
The test on a flat paper puts the problem in evidence, and this case is a problem on these days where companies ask you to send them a copy of documents through your phone.
Aside on that, that test puts in evidence that when not in a "perfect" sheet-of-paper case, the pictures of people and objects you take in the problematic distance are not sharp as they should be, and sure people will like them as they do not have enough experience and/or comparison of the shot taken with a good quality phone.
And the problem is not on the corners as you say, but starts near the center, and is awfull in the corners.
I started to notice the problem on abasic object shot, where I noticed near the center a visible loss of quality compared to my old iPhones. And proceeded to reproductible tests with a flat paper.
Last, as far as I know, the Vivo X200 Pro Mini has same "dimensional" specs, and has half of the problem. And not the flat-paper one. So possibly not a full 100% hardware problem.