This happens because in the current version of Telerik, the IsTopmost of the ToolWindow that host the floating pane is set to True. That was needed for a related new functionality, but it brings a major behavioral change in the RadDocking control.

To work this around, you can subscribe to the ToolWindowCreated event of RadDocking and set the IsTopmost property of the creating ToolWindow to False.

private void RadDocking_ToolWindowCreated(object sender, Telerik.Windows.Controls.Docking.ElementCreatedEventArgs e)
{
    var window = (ToolWindow)e.CreatedElement;
    window.IsTopmost = false;
}