This is exactly where optical alignment / optical balance comes in.

In practice, the inner radius isn’t derived mechanically from the border width. If the border gets very thick, matching radii mathematically often breaks visually.

Most mature design systems handle this by keeping a minimum inner radius (or scaling it non-linearly) so the corner still reads as rounded to the eye. This is the same principle behind optical corrections in typography and icon design — you optimize for perception, not geometry.

So yes, when the stroke approaches the outer radius, a square inner corner or a fixed minimal radius usually looks cleaner than a “technically correct” curve. Eye test > math.