I see, so I made an assumption earlier based on my own experience and it may have been pointing me in the wrong direction. I haven't worked with the device name prompter in a while. Part of me is wondering if it would help or hinder to move the prompter action into Post-OS Installation just above the unattend injection? I'm not 100% sure on whether the prompter is adding your device name to an xml at the time (if so, maybe that's the issue -- your image deployment will more than likely wipe the location) or storing it temporarily until such a time as the unattend.xml is actually injected/called (in which case I'm completely off-base).
This particular troubleshooting doc might help; it goes into some detail on what logs will show you where there is an unattend failure). How to troubleshoot Device Name Prompter Not Changing Computer Name
Finally, the first comment from this article also points out a potential problem w/ device namer: How to rename Computers Using the LANDESK Provisioning "Device Name Prompter" Action