issue when adding an item to a many to many relationship .net core - entity-framework

Question

i have a course management app and i have an issue when i try to add a student to a course studentsList.
when save changes run the student list changes from a list which contains old elements+ the new element to a list with only the new element (the last student added) and then saves it to database.
using EntityFramework 5.0.0
relationship declaration and seeding in context OnModelCreating()
modelBuilder.Entity<Course>().HasMany<Student>(c => c.StudentsList).WithMany(s => s.Courses).UsingEntity(t => t.HasData(
new { CoursesCourseId = Convert.ToInt64(1), StudentsListStudentId="id1"},
new { CoursesCourseId = Convert.ToInt64(1), StudentsListStudentId="id2"},
new { CoursesCourseId = Convert.ToInt64(1), StudentsListStudentId="id3"}
));
models
public class Course
{
public long CourseId { get; set; }
public string CourseName { get; set; }
public IEnumerable<Lesson> LessonsList { get; set; }
public DateTime CourseStartDate { get; set; }
public DateTime CourseEndDate { get; set; }
public List<Student> StudentsList { get; set; }
public class Student
{
[Key]
public string StudentId { get; set; }
public List<Course> Courses { get; set; }
}
controller action
[HttpPatch("{id}")]
public async Task<IActionResult> RegisterStudentToCourse(long id, [FromBody] List<string> studentsIds)
{
Course course = await _courseRepo.GetCourse(id);
if (course == null || !studentsIds.Any())
return BadRequest();
IEnumerable<Lesson> list = await _lessonRepo.GetLessons(id);
List<Lesson> lessonsList = list.ToList();
List<Student> studentsList = course.StudentsList.ToList();
foreach (string studentId in studentsIds)
{
Student student = _studetRepo.GetStudent(studentId);
if (student == null)
return BadRequest();
if (studentsList.Contains(student))
continue;
//HERE IS THE CALL TO THE REPOSITORY ADDSTUDENT
await _courseRepo.AddStudent(id, studentId);
foreach (Lesson l in lessonsList)
{
Attendance attendance = await _attendanceRepo.CreateAttandance(studentId);
await _lessonRepo.AddAttendance(l.LessonId, attendance);
}
}
_context.SaveChanges();
return Ok();
}
repository function
public async Task<bool> AddStudent(long id, string studentId)
{
Course course = await _context.Courses.Include(c => c.StudentsList).FirstOrDefaultAsync(c => c.CourseId == id);
Student student = await _context.Students.FirstOrDefaultAsync(s => s.StudentId == studentId);
if (course == null || student==null)
return false;
course.StudentsList.Add(student);
return await _context.SaveChangesAsync() > 0;
}