Shangguan Xi disappeared for 2 years because of a misunderstanding.
Xia Chengxuan looks like Shangguan Xi but has a completely different personality. He shares memories with An Cai'er and refuses to admit that he is Shangguan Xi.
An Cai'er waited alone for two years, thinking that Shangguan Xi had finally come back, but the other party kept insisting that he was Xia Chengxuan. Every moment they get along reveals that Xia Chengxuan is Shangguan Xi.
Shangguan Xi, Xia Chengxuan, who are you?