This flatness comes from the aspect ratio of your chart.
The current implementation of rounded corners doesn't take into account ratio of your chart's frame.
For example, if you give your chart the height and width of 500 & had a cornerRatio value of 1, your columns will look perfectly rounded.
If you change the height of that chart to 250, keeping the width at 500, this is where you will start to see the flattened rounded corners.
I hope this explanation helps clear things up.
I have forwarded your interest on this topic to our developers who will investigate this further.
Let me know if you have any questions.