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

If an iOS device happens to be displaying the Enter Passcode screen while the Clear Passcode command comes down to the device, the passcode does not clear until the screen is turned off and back on again.  Once this is done, however, the passcode is cleared and the user can continue using the device.   This is a function of the operating system.

To prevent this issue, the user should exit from the Enter Passcode screen before the Clear Passcode command is issued.