@nick So the next thing I would do is to update the firmware on the mobo just to be sure that isn’t it.
Then try the snponly.efi boot loader. You will need to update your dhcp server for this. The snponly.efi boot loader uses the driver built into the uefi nic. Its akin to undionly.kpxe for bios. The ipxe.efi boot loader is more like a linux kernel in that it has all of the common and known drivers built in, most but not all.
If that fails then here is how to build the latest version of iPXE. Note the build number on your current version of iPXE. Its the hex characters in the brackets after the version number. https://forums.fogproject.org/topic/15826/updating-compiling-the-latest-version-of-ipxe
If this doesn’t work then we will need to get wireshark on a witness computer (second computer on the same subnet as the pxe booting computer). This is a bit more involved so lets start the easier stuff first.