To reproduce:

Create a RadForm and add a RadButton. Open the Element Hierarachy Editor and set the ShowBorder of the RadButtonElement to false. Another way is to set the ShowBorder property through the Properties section in Visual Studio. Save the form, close it and reopen it. You will see that the border is shown  and the property is set to true.

Workaround:

Set the value at runtime:

this.radButton1.ButtonElement.ShowBorder = false;