What you actually get: a 2px rule on the left, rounded on the right.
The mirror image: a 2px rule on the right, rounded on the left.
Far corners of 12px and 6px: the smaller one is the radius.
A 4px rule reported before, as a stripe with no radius, and still does.
A 2px rule on a square box is the older callout convention.
Rounded only along the stripe: the far side is square.
One far corner under 4px reads as square.
Far corners with no vertical radius are square, whatever their horizontal one.