To reproduce:

Add a RadScrollablePanel and a button at a location of 500, 1000. Start the app and click the panel(not the button) you will see that you cannot scroll the panel.

Workaround:

Subscribe to the MouseWheel event of RadScrollablePanel:

void RadScrollablePanel_MouseWheel(object sender, MouseEventArgs e)
{
    typeof(Control).GetMethod("OnMouseWheel", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).Invoke(scrollablePanel.PanelContainer,
        new object[] { e });
}