It is! Your issue is that the scaled version of the rounded rectangle has corner radii not parallel to the outer ones.
I’ve whipped up an example in Onshape: The correct way is to have an offset between the outer and the inner wall (here 2mm). If the outer corners have a 5mm radius, and the wall is 2mm thick, then the inner radius has to be 5mm - 2mm = 3mm.
The way with scaling, the inner radius will be also just scaled, so when scaling a 50mm rectangle to 46mm, any 5mm corner will be scaled to 4.6mm – and that’s no longer parallel.