if the template is scheduled it will start on a PXE or USB boot. The scheduled task has priority. If you cancel or delete the scheduled task you will get the authentication UI.
If the PXE rep and the machined being PXE booted are on the same subnet I would make sure the DHCP options are set correct.
You are correct.. the machine being PXE booted does not need to be in a domain.
if all else fails load wireshark on the PXE rep and trace the traffic. This will let us know where the negotiation issue is.
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