When the RadButton is used as a toggle button and some of its toggle state buttons are set with PrimaryIconCssClass property the icons might not be displayed if the following conditions are met at the same time:

-AutoPostBack property of the RadButton is set to false

-One of the togglestate buttons is selected and its PrimaryIconCssClass property is not set.

The issue does not affect the functionality of the button, just the displaying of the icons in the togglestate buttons.