If you change the source during the tab control is not visible and if during load it didn't had items the SelectedIndex property is not updated properly. You can just update the SelectedIndex at your will and the respective tab will be selected. Also if item source is provided during the tab load method then you will be able to change the source and first tab will be selected.
This is a bug and you can track its state in our PITS
under the following title: SelectedIndex not updated when Visibility=Collapsed and ItemsSource is Changed .
All the best,
the Telerik team
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