From what I recall, yujin broke the contract and left korea on his own without informing Aaron. And Aaron did try to find him on the airport but yujin's plane already departed and yeah that's about it.
[Spoilers for those who haven't read, so please don't read the below if you haven't caught up to season 3 start]
Yujin said he wouldn't leave and Aaron decided to trust him, but Yujin ended up leaving and is trying to avoid Aaron. Aaron apparently put trackers on Yujin but Yujin left the country with only the clothes on his back, so Aaron had no clue where he actually flew off to. The reason Yujin left is because Yujin's father broke to the news that Yujin is the son of him and so I think Yujin only left because he felt he was a burden on Aaron with all the rumors spreading about how odd it was that the son of a man who tried to harm/kill Aaron was dating him. Basically the news thinks Yujin has ulterior motives, so he fled for Aaron's sake.
I came back from my yaoi retirement to read the next season can someone remind me of what actually happened because I can’t remember if Yujin was actually kidnapped by Aaron or Yujin just skipped the fuck out of town for no reason