To reproduce:
- set your windows taskbar to auto-hide. 
- Add RadForm to a blank solution.
- Change the forms behaviour with the following one:

RadRibbonFormBehavior radRibbonFormBehavior1 = new RadRibbonFormBehavior();
this.radRibbonFormBehavior1.Form = this;
this.FormBehavior = this.radRibbonFormBehavior1;

- When you maximize the form the taskbar won't show like in the normal case.