Change in child’s personality after brother’s shared custody arrangement?

A brief history.
2 children (7 years male, 9 years male)

Child 7 lives with the child's mother and has 24-hour visits to the child's father every four weeks.
Child 9 lives with the child's father and visits the child's mother for 3 days every 8 weeks.

Now the following problem;
A 7-year-old and a 9-year-old child go to the same school and see each other for about an hour a week, spread over 5 days. Now the 9-year-old child's weekend visitation with the child's mother is over, and the 7-year-old child is exhibiting severe behavioral problems REPEATEDLY.
(A 7-year-old child is a friendly, helpful, and loyal child. A 9-year-old child is the opposite, stealing from the family, being cheeky, and disregarding all rules and statements.)

Now we always receive reprimands one day before a change of school weekend and for two or three days afterward, starting with school refusal, breaking school rules, using bad language, and arguing. All of the problems occur exclusively at school. At home, the 7-year-old child is the sweetest child.

The school performance of a 7-year-old child suffers as a result.
Has anyone experienced something similar? Do you have any tips? Do you have any advice?

ps: Psychologist appointment is still taking a long time, pediatrician says to observe.