This issue occurs when an (APNs) Enterprise Developers certificate has been applied to the NotifyMDM organization.

The iOS MDM API defines a device by the UDID, not the username registered to the device.    Therefore, when a new user registers on a device previously registered to another user, the device tries to process commands from the first user account.

This can be remedied by using the Clear Device Registration option for the first user before assigning the device to and allowing another user to register it to the account.  Disabling the first user will also remedy this issue.