Or, someone's spreading misinformation about him on purpose to make his injuries seem worse. No one needs to bother trying to assassinate an heir that can't have heirs, he'd be set aside for the other princes to advance. Bide some time while he's recovering, then reveal it was some major BS when he's able to deal with stuff like attackers in the middle of the night again. Or, he's simply trying to annul the engagement for now because he has no idea the girl he found interesting is his fiance. Regardless, exaggeration of injuries could be useful in court politics I suppose.