
The kiss definitely made him recover some memories but faces are still blurry thats why he thought “what if it is not you”. He recovered bunch in that villa because many of what Hakyung does overlaps with his memory of the past. And NOVEL SPOILER…
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Hakyung will have a slip of the tongue when they get back home, telling YS, “maybe she contacted you but u dont remember her anymore”. HY didnt think much of it, but that weird call YS received in the past (when HYs Mom died) lingered in his mind all those years. So dots instantly connected, but he was still wary to tell anything to HY. (๑•ㅂ•)و✧
If you go back to ch 18, there is actually a hint there. Sis in law saying something to Jiyoung that it is ridiculous that Yoonsung hasnt noticed yet when he is usually always quick to catch on things. (≧∀≦)