Recently we have reviewed this item and would like to inform you that with the current implementation of RadDocking the observed behavior with the binding of IsHidden is considered as an expected one. When IsHidden is set to True the Pane gets removed from the visual tree and the Items collection of the PaneGroup - the same behavior could be observed with the native WPF controls when UIElement gets removed from the Items collection of another control (the "WPF ItemsControl" attached project demonstrates that). Because of that the Pane no longer inherits the DataContext of its parent as it is no longer present in the visual tree and the binding is not working.
In order the binding to be preserved and everything to work as expected the DataContext of the Pane should be set with the DataContext of its parent. You could check how that approach works in the attached project named "RadDockingWPF".