When longer string is used for HeaderText the text could not be wrapped. Also the issue occurs when using Label with LineBreakMode="WordWrap" inside the TabViewHeaderItem.Content:
When you have declared a RadTabView with a single item and you try adding items at runtime - the new TabViewItems are added to the Items collection of the control but the UI is not updated.
Allow different width tab headers. For example, 200, 100, 100. Available in minor release 2018.2.727.250. It will also be available in the R3 2018 release.