So anyone knows whats the truth behind his ex bf cheating?

I can guess...
I think he's from some sort of a reach complicated family and if he got too involved w her he might have had to hurt her later ( for all kinds of various reasons like he has to have an arranged marriage or whatever and so on) so the genius that he is he came up w a plan to make her leave on her own and start hating him hence he faked cheat.
It's just a guess tho.

spoilers
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i looked at raws and the heels belonged to an older lady which im assuming is his mom (this is in ch.27: https://tkor.toys/꽃_피우는_남자_27화.html ) so i dont think he cheated on her