[RPi4] Network boot IPv6 not working properly

[vintozver]

Describe the bug

Pure IPv6 netboot setup
I see the config.txt fetched, and attempts to fetch start4x.elf, but fails with "Firmware not found".
TFTP server reports incomplete transation.

Steps to reproduce the behaviour

1. set up radvd
2. set up kea-dhcp6
3. flash/update EEPROM with the config mentioned (netboot, IPv6)
4. tshark on LAN port to collect NETCONSOLE logs
5. unplug-replug to initiate the network boot

Device (s)

Raspberry Pi 4 Mod. B

Bootloader configuration.

[all]
BOOT_UART=0
WAKE_ON_GPIO=1
ENABLE_SELF_UPDATE=0
BOOT_ORDER=0xf241
USE_IPV6=1
NETCONSOLE=6665@169.254.1.1/,6666@/

System

https://forums.raspberrypi.com/viewtopic.php?p=2262886

Bootloader logs

https://forums.raspberrypi.com/viewtopic.php?p=2262886

USB boot

No response

NVMe boot

No response

Network (TFTP boot)

https://forums.raspberrypi.com/viewtopic.php?p=2262886