JSON styles can't apply rounded corners, how to fix it?

I'm using the Media and Text block and applying rounded corners to the images via JSON based style settings. However, in the style attribute of the block, setting the "border.radius": "20%" does not take effect and the image remains in its original shape.

I tried different values (e.g. "20px", "50%") but none of them worked.

What Causes JSON Styles Not to Apply Correctly border.radius?