Spoiler :
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Just some more wait.....
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To put it simple, their past communication was totally suck. Tia was so stoic back then so the prince assumed her only after the empress position. He initially didn't hate her actually, but there's some problem in their childhood that traumatized both of them and tia was so shocked she forgot it and it (seems like) changed her personality. The prince also hurt, but tia's new personality didn't sit well with him plus he also had another problems so he ended up misundertood tia. While his past action indeed were too cruel, tia past personality also at fault for their relationship. The guy already had so much problem in his hands, and tia (outside) indifference pissed him so much (still, his action was too much though, like he throwed all his anger toward tia, what a total bastard)
That's why when tia current personality changed and people can see through her more clearly, the prince warmed up on her and started to take interest because current tia's action was so unpredictable and it made him curious rather than angry like before. Current tia personality also seems like doesn't much different compared to her childhood before amnesia so it also bring nostalgic feeling for him.
That's what i got until ch. 70 (latest chapter updated in my first language), only 2 or 3 chapter behind original. Other people probably can see it differently though. I could say so far the prince still quite nice (and surprisingly have some affectional feeling toward her). Honestly if you able to treat the past prince and the current prince as different person, this manhwa actually quite enjoyable and won't offended you so much lol

Ok so i saw a ton of people hating on the blue haired prince, and ok he wasn't the best, he was far from the best but i already looked at the raw manga and although i couldn't really understand it bc i don't speak korean, with the help of google translate XD i was able to at least understand that he had his own reasons for hating her. I am gonna stop here bc i don't want to spoil anything