If you PXE boot UEFI hardware into an NBP (Network Boot Program) that is not UEFI compliant you will not be able to boot in native UEFI mode and your only choice would be the "Legacy Mode"
try to see what WDS is providing as NBP and check if this is UEFI or Legacy.
WDS detects your client's architecture and BIOS/UEFI mode using a DHCP option provided by the client. WDS then decides what NBP to send to the client. Faulty firmware might be also triggering your problem if the architecture/mode is not correctly declared by the client.