I'm going to say that he was probably the one who initially said he wanted to marry MC, and then forgot about it like an idiot. He's probably the type that gets bored quickly and then tries to change the narrative to convince everyone he never liked the thing in the first place.
I hope the dude is just under a lot of stress from assuming his new duties and been especially cold to her due to hormones or smh. I'd rather see a silly redemption story rather than "he was a jerk to me once and now I am dating an Ace-kun over there" storyboard #99.
thank you for the TL!! honestly, i like a good redemption so im cheering for the prince!! it seems that the stress of taking over as crown peince changed him (+ did he even have time to mourn his brother?) poor guy
theres only a handful of ways these writers write these characters. im putting 10 on the girl is really talented and prince boy got jealous of and/or angry at his own inferiority