Why does corner rounding change when resizing a box?

When I create a box and set the bottom corner rounding to 38 pixels, and then change the height of the box to anything under 69 pixels, the corner rounding begins to decrease (see orange in image below), even though the top of the box is above the top of the rounding of the bottom corner…

rounded corner

Can’t quite replicate that setup, but the idea is the radius is at most half the height.