As a work-around until this issue is resolved, we could use the loaded event of the RadTabControl and set width to the buttons in the ScrollViewer.

private void TabControlMain_Loaded(object sender, RoutedEventArgs e)
{
    var tabControl = sender as RadTabControl;
    var leftScrollButton = tabControl.ChildrenOfType<RepeatButton>().FirstOrDefault(x => x.Name == "LeftScrollButtonElement");
    var rightScrollButton = tabControl.ChildrenOfType<RepeatButton>().FirstOrDefault(x => x.Name == "RightScrollButtonElement");
    if (leftScrollButton != null && rightScrollButton != null)
    {
        leftScrollButton.Width = 17;
        rightScrollButton.Width = 17;
    }
}