The quirk with Corel Draw is that if you transform the rectangle (stretch it afterit's in position) the corners won't round proportionately.
To fix this problem:
1. Draw a rectangle and stretch it into the position and size you want.
2. Turn on "snap to objects" (ctrl+J, workplace, snap to objects)
3. Draw another rectangle in the same position as the last.
4. While the new rectangle is still highlighted, hit "ctrl + page down" to send it to the back.
5. Click away from the objects to deselect all objects
6. Highlight the first rectangle and delete.
7. Highlight the new rectangle and continue with corner rounding, the new rectangles corners will be rounded evenly.