There is an existing group in Active Directory with existing members. This group is converted to a Dynamic Group with the include group membership option, but some or all previous members do not appear.
Alternatively, a new or existing Dynamic Group may not display some members which are expected.
Expected membership could be blocked due to a setting within the Built-in Policy - Dynamic Groups policy.
One or more of the expected members were Deprovisioned and the Undo-Deprovisioning action was not properly performed.
Right-click on the User object and check to see if it shows an action to Deprovision or Undo-Deprovision. If it shows the Undo-Deprovision action, then this User object was previously Deprovisioned and this status was not correctly rolled back using the Undo-Deprovisioning action.
Complete the Undo-Deprovisioning process in order to correct this partial status.