Thank you for writing.
This behavior is indeed strange however, this XAML alone is not enough to diagnose the real cause.
I think you'd be interested to know that RadWindow uses a hack to improve its performance when it is in full screen mode. When IsFullScreen is true, RadWindow waits for its OpenAnimation to finish and then sets the Opacity property of all its siblings to 0. This usually decreases the fill rate by a huge amount and therefore provides a performance improvement. You can also simulate the IsFullScreen property by setting the PlacementTarget property of RadWindow to the layout root panel (or even the page) and then set the WindowSizeMode property to FitToPlacementTarget. This will avoid the opacity hack.
I hope the information below might give you a hint how to find the cause of the issue. If you still need assistance, please provide a small sample application that demonstrates the strange behavior so that I may assist you further.
the Telerik team
Let us know about your Windows Phone 7 application built with RadControls and we will help you promote it. Learn more>>