Remove Manager does not work with external DS repo (using the UI or PATCH via curl)

It doesn't seem like we can Remove Manager from a managed/user in the Admin Console or from running a curl command.

Steps to reproduce: 

Install DS 6.5.3 and populate DS with users for testing (I added 2k on DS installation).
Install IDM 6.5.0.2 and set it up with the External OpenDJ/DS as the IDM Repository.

Confirm the above is setup and you can login to IDM as openidm-admin and a user (eg. user.106/password).  

1. Login to IDM as the openidm-admin and simply Add a manager to a user and save the change. 
2. Then try to remove the manager.

Admin Console: 

  MANAGE > USER, NAME! Details TAB!
     and locate the "x Remove Manager" button just above Preferences section.

CREST command:  

$ curl --header "Content-Type: application/json" --header "X-OpenIDM-Password: openidm-admin" --header "X-OpenIDM-Username: openidm-admin" --data '[{"operation":"remove","field":"/manager"}]' --request PATCH "http://idm.example.com:8080/openidm/managed/user/6a0728b8-6f7e-40a9-ab68-19cd48e4f841"

Expected Results: Manager removed, and field is reset to empty (NULL).

Actual Results: Manager NOT removed, and field remains the same as
    before attempting to remove or NULL out the Manager field.