This issue is not just specific to tabPanel. Anytime the extendedDataTable is hidden (when the page loads) and then it is made visible later the table can not display the data (table is empty at the load time).
In this particular case if we load a page with two tabs. Furthermore if we have one extendedDataTable in each tab (the first tab shows and the second tab is not shown at first). In this case once the user clicks on the second tab and tries to populate the extendedDataTable (via any action). Then the table is not rendered properly. A sample project has been provided at the following link that further describe this issue. You can download the eclipse project attached at the end of the following message to reproduce this issue.