I’d say the top right corner does indeed have a radius, but you can’t see it because the scrollbar does not stretch up to the header row. This happens when you set fixed_rows={'headers': True}, which makes the scrollbar appear outside of the table instead of inside, causing a gap on the right side of the header (and the gap is where the radius would be).
Not sure if there’s something you can do about this. I’ve tried it without the fixed headers but the curved scrollbar looked strange…