If you use the code from the following help article: http://docs.telerik.com/devtools/aspnet-ajax/controls/tilelist/how-to/responsive-layout-and-dynamic-height#responsive-layout-and-dynamic-height and set ScrollingMode property of the RadTileList control to Accelerated after re-sizing the window it is possible to scroll vertically. http://screencast.com/t/KtCjTSAy2558
This problem also occurs in the normal scrolling mode btw.