At some point, you can find that a student is not linked correctly to a base group.
Then, you can link the student to the correct group manually. Access the class group where the student is > 3 vertical dots > Base group > Link students.
In Base Group you can find other options like Import students or Unlink a group.
When you select Link students you have to select the unlinked student > select the base group > select the student from the base group related to the student.