To reproduce:
Open the attached project and press Ctrl+ Tab in the first text box. Since the textboxes are accepting the Tab, Ctrl + Tab should move to the other control.


Workaround
// RadTextBox
private void TextBoxItem_PreviewKeyDown(object sender, PreviewKeyDownEventArgs e)
{
    if ((e.KeyData & Keys.Tab) == Keys.Tab && (e.KeyData & Keys.Control) == Keys.Control)
    {
        e.IsInputKey = true;
        var item = sender as RadTextBoxItem;
 
        this.SelectNextControl(item.HostedControl, true, true, true, true);
 
    }
}
//RadTextBoxControl
private void RadTextBoxControl1_PreviewKeyDown(object sender, PreviewKeyDownEventArgs e)
{
    if ((e.KeyData & Keys.Tab) == Keys.Tab && (e.KeyData & Keys.Control) == Keys.Control)
    {
        e.IsInputKey = true;
        this.SelectNextControl(sender as RadTextBoxControl, true, true, true, true);
 
    }
}