Some additional inputs:
I'm running the boot from a QEMU VM. In the QEMU logs I see the following error during HTTP boot:
Error: Server response timeout.
BdsDxe: failed to load Boot0005 "UEFI HTTPv4 (MAC:0C2E9A0E3600)" from PciRoot(0x0)/Pci(0x3,0x0)/MAC(0C2E9A0E3600,0x1)/IPv4(0.0.0.0,0x0,DHCP,0.0.0.0,0.0.0.0,0.0.0.0)/Uri(): Not Found
I've also raised httpd server logging to trace6 but could not find anything pointing to a problem on this side. I'm only able to observe an error after the UEFI terminated the session, which is expected.