While that is what the game says has happened, the war screen clearly shows that Tlemcen can't sign a separate peace, presumably because they are a vassal of Castille. This looks broken to me, much like how you can get call for peace when you are not the warleader and you'd love the AI to take the victory you've won for it.Tlemcen has unconditionally surrendered to you. Separate peace them and the call for peace will go away.