@Wayne-Workman That’s it. Thanks heaps for finding this! I thought I’d searched for \003 but obviously I didn’t - not properly at least.
You recommended to use undionly.kkpxe as the “quick fix”.
That’s not the whole truth I’m afraid! As it does not load a binary from the TFTP you need to use other methods like USB booting (rom-o-matic.eu, settings in the wiki but output format iso/usb). Or you could burn iPXE into the NIC ROM (haven’t done this myself yet so I cannot give any instructions). Why I did recommend undionly.kkpxe is because there is a known issue with a particular realtek NIC. As the OP didn’t let us know any machine or NIC details I am not sure if this is the same one. But I guess this is a very rare issue. So possibly it’e exactly the same NIC.