Most of the controls exposed in RadControls suite have built-in support for Visual Style Builder, which requires no aditional changes in code to apply themes and behaviors. Unfortunately RadPanel does not posses such capabilities, which is the reason for the strange behavior in design time after editing with the VSB. Fortunately this problem should not appear after reopening the designer. You may also need to re-add the control to the form's controls collection (add such a code line, say in InitializeComponent). You also have the option to use the UI element editor in design time to quickly customize a given instance of RadPanel control. We hope this will help until we add this functionality in our next version.
All the best,
the Telerik team