@nec Good find, but I would also say that you are missing the boot-file (or whatever its called) at the pool level. That is the bootp part of the pxe booting. When you look at the pcap from the witness computer on the same subnet as the pxe booting computer. You should see both sets of values set.

OK, I get that, though I must admit that the config file I’ve shown here is sufficient and working in that particular context.

I have no idea what Kea dhcp server is but the config file looks similar to the linux standard ISC-DHCP dhcp server.

ISC started the development of Kea in 2014, ten years ago, with the objective to replace the venerable ISC-DHCPD.
As stated here, ISC has set the end-of-life of ISC-DHCPD in 2022.

(On a personal POV, it broke my heart)