@NeedLessMacs So far we only seemed to have this issue with Sierra and later versions. See here:
It seems as if the ones that came with Sierra (10.12) pre-installed will not work with iPXE while the ones that had El Capitan (10.11) are OK.
Possibly a Sierra update came out that also broke it. Or we are looking at a different issue here. So far I have only heard of onboard NICs having that issue but if it’s the exact same chip could also be a problem with the thunderbold adapter.
Please read the whole iPXE from above and start by compiling your own debug iPXE binary to test.