In part 2, everyone knows Uk as someone who was revived through the ice stone and who has the ice stone. They called him a monster cos he was dead and revived. If they knew that Uk was the king’s star, they wouldn’t avoid him.

Master Lee, Uk, Naksu, Park Jin, Jin Mu & CP were the only ones who knew that Uk was the King’s Star even in part 2.

In ep 19 of part 1, at 39:30 min, they talked about the King's Star.

Uk told the CP to take the credit after the rain.

Mages from Unanimous were bowing down to the King's Star. Uk is quick-witted so there is no way that he doesn't know he is the real king star. CP knew it right away too. That's why he looked back.

 Muhis:

I don't think he knows that he is the late King's son, spiritually

Yes. I should also make who knows Uk is the Late King's son, spiritually.

 Zdzislawa:

Does Uk know about himself? I was curious about it, how it was never mentioned by Uk or with Uk regarding the king's star. 

Yes. Uk knows and he never mentions cos he is quick-witted.

He just wanted the CP to keep taking credit and feel the insult/burden for what happened to MD. Uk is suspicious of CP for being on Jin Mu's side after MD's death.