@richard-nihells You change the dhcp option 67 {boot-file} to that name.
So you are using 1.4.4. That’s good so you have a current version.
When its pxe booting, do you seen anything that says NBF? What your describing really sounds like a computer in uefi mode, being sent a bios kernel.
Just so you know bios (legacy) mode requires undionly.(k)kpxe kernels, where uefi mode requires a uefi kernel ending in .efi, like ipxe.efi again that would be set in your dhcp boot options.