Why First parent model reference is getting set as null when adding the same child to second parent using `ObjectBox` in flutter?

Question

I have two objects one parent model have ToOne relation and a child model with ToMany relation back to the parent. Now if I have two parents who are setting their relation to child then the first parent's reference to child is getting null when the second parent reference is set. Also, if I remove the id field from the child then both parents child is stored in the database.

Suppose I have a class like

Student{
  String id;
  final teacher = ToOne<Teacher>();
}

and another class Teacher as

Teacher{
  @Unique(onConflict: ConflictStrategy.replace)
  String? id;
  @Backlink()
  final students = ToMany<Student>();
}

so now if I try to do something like

student1.teacher.target = teacher1

followed by

student2.teacher.target = teacher1

then if I try to check for student1 then its teacher is getting null. Why ?