How can Jason remember Lian if he never met her? The closest he ever got to know her was when he joined the Titans on a mission after Jade and still, he never met Lian or didn't even know her name, just that Roy had a daughter ( important to note that this happened when Jason was like 12 years old).

Roy couldn't have told him about Lian during their time with the outlaws, she didn't EXIST. (later was revealed that everyone was brainwashed by Dr. Manhattan and lost their memories of 5 years of their lives but she was still erased from canon and didn't exist). And till the last time they saw each other, Lian still didn't exist. This is the first time they meet since Roy (and Lian) came back. There's no way he could have known about Lian.