Context menu highlighting broken

We are using the ribbon ContextMenu class for regular (non-ribbon) context menus, and have found a problem with the highlighting. When the menu is close to the right edge of screen, causing sub-menus to double back on themselves, the highlighting doesn't work if there is another menu under the active sub-menu. If you move the mouse to a part of the sub-menu that isn't over a parent menu, the highlighting starts working.

See here for a screenshot http://imgur.com/OY1Andc Though you can't see it, my mouse is over Sub Menu Item 1 in the screenshot.

To repro, paste this code into a sample app, then move the window close to right such that the first sub-menu appears to the right of the top menu, but the second sub-menu appears to the left (over the top menu)

<Grid Background="LightGray">
    <Grid.ContextMenu>
      <ribbon:ContextMenu>
        <ribbon:Menu>
          <ribbon:Menu.Items>
            <ribbon:Button Label="Menu Item 1" />
            <ribbon:Button Label="Menu Item 2" />
            <ribbon:PopupButton Label="Sub Menu A">
              <ribbon:Menu>
                <ribbon:PopupButton Label="Sub Menu B">
                  <ribbon:Menu>
                    <ribbon:Button Label="Sub Menu Item 1" />
                    <ribbon:Button Label="Sub Menu Item 2" />
                  </ribbon:Menu>
                </ribbon:PopupButton>
              </ribbon:Menu>
            </ribbon:PopupButton>
            <ribbon:Button Label="Menu Item 3" />
            <ribbon:Button Label="Menu Item 4" />
          </ribbon:Menu.Items>
        </ribbon:Menu>
      </ribbon:ContextMenu>
    </Grid.ContextMenu>
  </Grid>