I think he is just asking specifically about Flexi. Actually Illustrator, Corel, and Flexi can all do this just fine.
In Flexi when you initially make your rectangle it does it perfect but once you select something else then try to go back to the rectangle/square and just select it, grab a side and try resizing it then it will distort the rounded corners. To make sure this does not happen, when you select a rectangle with rounded corners, then you must now select the rectangle tool pallet (I always have the design central pallet open on screen). Once you have done this then the control anchors on the rectangle change and you can change its shape/size without any distortion of the corners. The attached shows what I am talking about. You have to also do this with text and when trying to edit certain other things - you have to select whatever it is then click on the correct tool for editing whatever it is.
No idea how it works in Corel since I don't use that but I know it also does it just fine. In Illustrator you do NOT want to use the round corners function of the rectangle tool itself or you will have the same problem. In Illustrator make your rectangle with normal square corners then with the rectangle selected go to the top menus>Effects>Stylize>Round Corners and set the corner radius you want. When the rounded corners are applied and an "effect" they will not distort when your later try to resize or scale the rectangle.
This is what you want select in Flexi after you select the rectangle....