One of my junior researchers has been hated by professors without any theoretical reason

An assistant professor leaked out to me that one of my junior researchers has been hated by my professor.
The incident can trace back to the times of deciding which laboratory he should be assigned to. Then, my professor imposed him on other assistant professors by saying that he should be banished from my laboratory.
But, the fact is that the junior researcher did nothing as to be blamed. The problem was that my professor's dementia made the professor assume that the junior researcher hadn't submitted documents before a deadline.
The professor's illusion spread the assistant professor.
Though the junior researcher wants to enter graduate school, the assistant professor told me "He had better be engaged in job hunting or go to another graduate school."
I was disappointed with the attitude of my professor as to cause discrimination against the junior researcher, who study earnestly as far as I see.