Can non aggregate-root hold a reference for another non aggregate-root?

If I have two aggregates like this:

First Aggregate :

• WorktimeRegulation (Root)
• Worktime
• RegulationEnrolment

Clarification with data:

WorktimeRegulation :

 public class WorkTimeRegulation : Entity<Guid>, IAggregateRoot
    {
        private WorkTimeRegulation()//COMB
       : base(Provider.Sql.Create()) // required for EF
        {
        }
        private WorkTimeRegulation(Guid id) : base(id)
        {
            _assignedWorkingTimes = new List<WorkingTime>();
            _enrolledParties = new List<RegulationEnrolment>();
        }
        private readonly List<WorkingTime> _assignedWorkingTimes;
        private readonly List<RegulationEnrolment> _enrolledParties;
        public string Name { get; private set; }
        public byte NumberOfAvailableRotations { get; private set; }
        public bool IsActive { get; private set; }
        public virtual IEnumerable<WorkingTime> AssignedWorkingTimes { get => _assignedWorkingTimes; }
       public virtual IEnumerable<RegulationEnrolment> EnrolledParties { get => _enrolledParties; }
        //...
    }

Id|    Name            |   NumberOfAvailableRotations|  IsActive 

 1|    General Rule    |          2                  |    true   

Worktime :

public class WorkTime : Entity<Guid>
    {
        private WorkTime()
      : base(Provider.Sql.Create()) // required for EF
        {
        }
        private WorkTime(Guid id) : base(id)
        {
            ActivatedWorkingTimes = new List<WorkingTimeActivation>();
        }
        private ICollection<WorkingTimeActivation> _activatedWorkingTimes;

        public string Name { get; set; }
        public byte NumberOfHours { get; set; }
        public byte NumberOfShortDays { get; set; }
        public Guid WorkTimeRegulationId { get; private set; }
        public virtual ICollection<WorkingTimeActivation> ActivatedWorkingTimes { get => _activatedWorkingTimes; private set => _activatedWorkingTimes = value; }
        //....
   }

Id|  Name   |   NumberOfHours| NumberOfShortDays |WorkTimeRegulationId 

1 | Winter  |     8          |    1              |    1
2 | Summer  |     6          |    0              |    1

Second Aggregate :

• Shift (Root)
• ShiftDetail
• ShiftEnrolment

Clarification with data:

Shift :

  public class Shift : Entity<Guid>, IAggregateRoot
    {
        private readonly List<ShiftDetail> _assignedShiftDetails;
        private readonly List<ShiftEnrolment> _enrolledParties;


        public string Name { get; set; }
        public ShiftType ShiftType { get; set; }
        public int WorkTimeRegulationId { get; set; }
        public bool IsDefault { get; set; }
        public virtual WorkingTimeRegulation WorkTimeRegulation { get; set; }
        public virtual IEnumerable<ShiftDetail> AssignedShiftDetails { get => _assignedShiftDetails; }
        public virtual IEnumerable<ShiftEnrolment> EnrolledParties { get => _enrolledParties; }
        //...........
   }

Id|  Name      |  ShiftType  |  WorkTimeRegulationId  | IsDefault 
1 | IT shift   |  Morning    |    1                   |  1 

ShiftDetail:

  public class ShiftDetail : Entity<Guid>
    {
        public Guid ShiftId { get; private set; }
        public Guid WorkTimeId { get; private set; }
        public DateTimeRange ShiftTimeRange { get; private set; }
        public TimeSpan GracePeriodStart { get; private set; }
        public TimeSpan GracePeriodEnd { get; private set; }
        public virtual WorkTime WorkTime { get; private set; }

        private ShiftDetail()
        : base(Provider.Sql.Create()) // required for EF
        {
        }
        //..........
   }

ShiftId  WorkTimeId shift-start  shift-end   
  1          1        08:00        16:00
  1          2        08:00        14:00

My questions here:

• Is it okay for non aggregate-root (ShiftDetail) to hold a reference for another non aggregate-root (WorkTime)?

• The domain expert clarify that: To create a valid shift then we should have a shift detail for every worktime related to a specific worktimeRegulation. And can't update the number of workhours in worktime if there's reference in shiftDetails. The previous example shows that we have two worktimes(winter,summer), So we have a shiftdetail for winter to stick with 8 workinghours and a shiftdetail for summer to stick with 6working hours. Now I feel that invariant of shift details controlled by non-aggregate root(worktime) How to force this invariant?

• According to the previous information do I make a mistake related to aggregates specifications?