When you have the TabStripPlacement property set to something other than Top and you try to hide the TabControl border by setting the BorderThickness property to 0. The border is still shown. If you have TabStripPlacement set to Top the border disappears. As a workaround set the BorderBrush property of the RadTabControl to "Transparent".
Hi, On 12 November 2018 (Monday), we are uploading our next LIB version (2018.3.1112) where the fix will be included. It will also be added in our official version – R1 2019, scheduled for the mid of January 2019. Best regards, Ralitsa Kumanova