DHCP Client sends GUID as unique ID and not MAC

Hi All!

Having some trouble with a client in our Network. We use DHCP MAC Reservations for our Clients, so they keep the same IP Address. One of our Clients keeps getting an IP Address of our free Range instead of the IP Address from his reservation. When I check the Address leases of our DHCP Server, the unique ID of the lease from the problematic Computer is the GUID, which you can see in preboot instead of the MAC Address like all the other Computers.

Does anyone know how to deal with this Problem?Cheers!

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EDIT: Clients are Windows 7 and 10. DHCP Server is Windows Server 2012.