Strict child matching, similar promotions. Sigurd was a Mage Knight, but Seliph was a Thief. Thief has only one valid promotion under the given rules (similar promotions requires same or greater bases + class skills), which is Rogue. That ends up breaking the other rule of retain mounts (I'm assuming units who promote into a mounted class from a foot class always promote into a mounted one if retain mounts is enabled - is that correct?).