Here is the scenario that leads to situation that seems to be bug in XamDockManager.

1) There is some application with XamDockManager as the content of the window.

2) There are several panes docked.

3) Two of those panes, are dragged to the bottom border, so a a result they are both placed in TabGroupPane. That should be the only two panes in that area.

4) Unpin those panes.

5) Save layout of XamDockManager to file.

Here are some of the most interesting parts of the saved XML file:

-- in contentPanes tag

    <contentPane name="pane1" location="Unpinned" lastDockableState="Docked" lastFloatingSize="40,15" lastFloatingWindowRect="496,608,48,43" lastFloatingLocation="496,608" lastActivatedTime="2010-03-09T13:27:42.4092838Z" unpinnedOrder="0" flyoutExtent="129" />
    <contentPane name="pane2" location="Unpinned" lastDockableState="Docked" lastFloatingSize="65,15" lastFloatingWindowRect="78,269,73,43" lastFloatingLocation="78,269" lastActivatedTime="2010-03-09T13:27:40.2686725Z" unpinnedOrder="1" flyoutExtent="129" />

-- in panes tag

    <splitPane name="Zbb61ea99393f4f8eb5ac908749ef1a8b" splitterOrientation="Vertical" location="DockedBottom" extent="125">
      <tabGroup name="Zc5225d7c35c64d128eae1996c44cfca9" selectedIndex="-1">
        <contentPane name="pane1" />
        <contentPane name="pane2" />
      </tabGroup>
    </splitPane>

6) Close Application.

7) Start application again and load layout. Both panes that were unpinned on the botton dock area are visible as unpinned, which is good. But, when I go with mouse to the headers of those panes, they do not show their contents, which is bad.

I've came to this issue by manually generating XML file for layout restore, but I also reproduced it by creating desired layout manually and using SaveLayout method of the XamDockManager.