CUPI Update PIN fails if User has a Role (e.g. System Administrator)

I have a customer attempting to change the voicemail PIN of a user through the CUPI API. They noticed that this fails if the mailbox has been given a Role such as User or System Administrator. The error returned is:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<ErrorDetails>
    <errors>
        <code>IMS_ERROR</code>
        <DetailCode>1030</DetailCode>
    </errors>
</ErrorDetails>

Is it expected that CUPI cannot modify privileged accounts? I suspect the answer is 'no' since this same API can also grant users those same roles. Is this a bug or are we missing an extra step for modifying privileged accounts?

For reference, this is being observed on Unity Connection version 11.5.1.12900-21. Here is the URL, headers, and body. I'm able to readily reproduce this through Postman.

PUT URL: https://<connection-server>/vmrest/users/<user-objectid>/credential/pin
Headers:
Content-Type: application/xml
Authorization: Basic [redacted]
Body:

<?xml version="1.0" encoding="UTF-8"?>
<Credential>
  <Credentials>050917</Credentials>
</Credential>

Hi Jonathan,

The password change and password setting modification for User PIN/Web application should be provisioned on Publisher server in Active-Active mode.  If the user has sys admin roles, then he can perform  those CUPI interface. Are you saying that a regular user can grant sys admin role to himself?

Regards,

David