What Cascade type should I use to keep existing relationships on manyToMany when saving on JPA?

Question

I'll try to make it simple:

I have an Employee and a Project entity with a many to many relationship like so:

public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @Cascade({})
    @ManyToMany
    @JoinTable(name = "employee_project_fabricio",
            joinColumns = @JoinColumn(name= "employee_id"),
            inverseJoinColumns = @JoinColumn(name = "project_id"))
    private List<Project> projects;
}

public class Project {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer id;

    @Cascade({})
    @ManyToMany(mappedBy = "projects")
    private List<Employee> employees;

Whenever I try to send a PUT request to my updateEmployee endpoint with a new list of projects, the old relationships that i already had saved gets overwriten with this new list. I want to keep the old relationships and add the new one that i just sent the request. I've tried already multiple types of cascades, but none of them seems to work.

This is my service method, just in case:

public EmployeeDTO updateEmployee(EmployeeDTO newEmployeeDTO) {
        Employee employee = toEmployee(findEmployeeById(newEmployeeDTO.getId()));
        List<Project> projects;
        Employee employee1 = updateEmployee(employee, newEmployeeDTO);

        if (newEmployeeDTO.getProjects() != null) {
            projects = projectService.createProjects(newEmployeeDTO.getProjects());
            employee1.setProjects(projects);
        }

        return toDto(employeeRepository.save(employee1));
    }