So it seems that the RectangleItem - the way radii are calculated is incorrect/inconsistent with standard design and CAD software (Figma, Adobe products, the entire CAD world, etc.) 
It seems that border lines for both Inside and Outside Modes (for Rectangle Item) radii corners use the "outside" to calculate radii measures.

The correct radii measurement is referenced from the rectangular corner, with Inside borders laying in offset inward, and Outside borders layout in offset outward - with both remaining in contact with the reference radii measurement. 

Here is a Figma file that correctly executes the radii (representing the graphic industry standard): 

https://www.figma.com/file/ezAG70VLi7qsseZNVe5mKl/Radius_BUG?node-id=0%3A1&t=Ay1xpSM8ShvcrRIW-1

Below I have attached some explanation illustrations (2 pngs) 

and a Qt Design Studio file that matches the Figma file above, that shows off the differences clearly...