With R1 2021 release the  the content of the first tab of the TabView is not visible on UWP.

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.

Under "normal" conditions, the user interacts with each tab in turn, finally getting to the last tab and submitting valid data, but under some conditions the tab control appears to not function correctly - clicking on the headers quickly will change which tab's header is highlighted, but the display content does not change. 

if you rapidly switch between random tabs immediately after the page is displayed and IsContentPreserved is True, the tabview content stops getting updated