Mmm, I think he never met her until the day the arranged marriage was finalised and usually they met a day prior before the engagement to make both of them closer or a day before the ceremony. Either way, he thought she was the daugther of the duke (?) and as crown prince, ofc he should he aware the duke family wasn't a good person, in addition the real daugther was not a good person too, arrogant to bone obviously she also made MC a sacrifice. If I were him in the past, including MC that was really innocent and only knew the surface not the depth, I'd also be wary of her and told the same thing because either he knew she was a duplicate or not she's from and a daugther of that duke. Just look at that, as they met not as political couple and she's not bearing that identity, he acted fine and even worried no hostile/cold treatment whatsoever
And yeah, there's no way he couldn't be the ML because MC said she actually also want to he empress tbh in this chapter, she even admitted she's no different to the real daugther, it's just currently she didn't wish to use/harm innocent people like what the duke's family
By only knew the surface not the depth I meant she thought she's blessed and people were good people as long as she didn't harm them and acted sweet + docile withour knowing business/
political issue could trigger people to commit a bad deed
She might have trauma for being one-sided love with her husband as all the time she's fakely pampered by mostly people surrounded her but the previous crown prince might also have nightmare for need to be wary of the duke family, for marrying with wrong family that obviously greedy for power (iirc all she wanted was the son/legitimate crown heir). Especially when a son was born, it meant the duke's power raised significantly, and when the real daugther was healthy to take her identity back, the nightmare of that prince which was being controlled like puppet came true.
Well, it's all my assumption cause usually that's what's happened in some manhwa but he looked normal compared to what MC's imagined from the past. And sorry for bad english ~