Thinking about it, I think Chunjin is in a later scene where Shaw's mother visits and in her arrogant manner she gives Chunjin cooking instructions.
"Chicken soup - with a beep straw."

While being debriefed, Marco in the film probably told his superiors about Chunjin but there is no clear indication of it and he simply drops out of the film, except that Marco and Raymond Shaw talk about him in a rather jocular manner. In the book Chunjin continues to be active. I thought it odd that the Soviets would put Chunjin in with Shaw because there is the danger that his presence at Yen Lo's session might trigger a memory in Raymond - as it does to Marco when the latter unexpectedly visits the apartment. Perhaps it was seen as a test of how effectively Shaw was programmed.
I think he was dropped from the later action in the film to simplify its already complex plot, and perhaps to signify an early victory over the brainwashing by the neutralising of one of those involved in it. In the book, Chunjin is hospitalised but insists on being discharged as quickly as possible so he can go back to work as Raymond's houseboy. He does not tell the hospital that if he loses the job, his superiors will shoot him. And he does go back to work and continue as Raymond's servant and chauffeur, including driving him to assassinations.
"Chicken soup - with a beep straw."

Thanks for your reply. But Marco is an agent of the United States government, in his capacity serving in the U.S. army with regular access to superiors, who eventually believe him, and a witness to Chunjin's involvement with kidnapping and brainwashing the platoon.
I just think it should have been explained what happened, if he disappeared from the hospital and then stopped working for Raymond.

In the book, he continues to be the houseboy and also a facilitator, for example he drives Raymond to the house of the senator and father of his wife, where he kills them both. In the film there is no indication that the US authorities know that Chunjin is a Soviet agent (in the book he is described as a lieutenant colonel in the Soviet security service) but they have an idea of this in the book. But he is not arrested, for whatever reason.
"Chicken soup - with a beep straw."