The attached patch is not a real solution to the problem but may be a hint to what is really going wrong. I am not sure why the inspectChild method in TabbedPaneFactory is being called twice for each child, but it is. The second time inspectChild is called, the title attribute is not there. This patch simply ignores the second call for each child and that makes the symptom go away.
Why is inspectChild being called twice for each child?