I think that personality disappeared because they all loved him at the end of this experience and that part of his personality didn't really love him so it wasn't a part of him anymore. I don't really know how to explain it
No, you're very right. In the end, Hyunjin had completely fallen for Doyoung. It'd be very odd to have a side of you that hates the person you love, right? You should never hate your partner. It's okay to get angry with them sometimes, negative emotions are included in any relationship, but hating them is something that just can't exist when you have unconditional feelings for someone. I'm currently in love with my best friend, and sure, I know about a lot of his flaws, but there's so much more room to figure out the rest, and I love all of his flaws. If I hated him in any way there would be a problem, and maybe you would even consider saying I never even loved him in the first place. So yes, a valid point, darling.
When doyoung saw all the personalities in hyunjin, there were only 11 of them. The author forgot about the hyunjin who died in the very beginning lol.