James Peltier writes:

Welcome to the world of UEFI. Certain UEFI versions have added additional support for things like the next-server option to actually be honoured. In some versions this _is_ in fact _ignored_ and you are expected to place the image on the server that answers the DHCP response.

I have been successfully booting both legacy BIOS and UEFI machines for at least 3 years and know the pains.

Thanks. This appears to be the case here. The machine is a Dell 11G server. I upgraded all the firmwares with dsu, BIOS was at latest already. After dsu, I upgraded the NIC option ROM with bootutil (I'm not using the builtin BRCM NICs, but an Intel X520-T2 card). That's all I can do on the hardware front. And it *still* shows the same behaviour.

Moving tftp server it is then ...