Thanks @kerrishotts - very insightful!
Indeed, I was working with native “live” rectangles. However, when I construct a rectangle with the Pen Tool, the rendering matches the other apps.
This brings the next comment; there is no way to glance at a rectangular shape to know how it’s been constructed. With much artwork still originating outside XD (most notably, Illustrator), the difference could be problematic for consistency.
However, it does answer the cases when this rendering approach takes place.
As you saw from the separate post Outline Stroke (border) and Offset Path functionality preview, we want to minimise times when outlining of the stroke (border) causes a change in appearance. In theory, we could handle live rectangles differently in the plugin, but I fear that’s a slippery slope.
Perhaps it’s a storm in a teacup - ie. not really important. For strokes (border) widths of 1 or 2px, you’d never see it.