In the novel, he was in Mo Xuanyu's body, so he looked different from his own body. But in the untamed, Wei Wuxian takes Mo Xuanyu's place with his own original body (Wei Wuxian's), that is why he wears the mask because people who once knew him, would still see that he is Wei Wuxian without the mask. So he still looks the same even after his resurrection in the untamed.
Also wow, Wei Wuxian straight up sacrificing himself to save Lan Zhan (cause he would had fallen if he kept holdinh wei ying bc the mountain side under him wouldve broken since Jiang Cheng stabbed into it and was breaking it) never ceases to make me so amazed and sad at the same time. The love runs deep with those two.