I have a many CM3L boards network booting happily with an old bootcode.bin (unsure of the version, but quite old).
I tried the newest available via Github for both the Master and Stable branches and the CM3L boards seem to hang after grabbing bootcode.bin from the TFTP server. Rolling back to a slightly older bootcode.bin allows them to boot successfully.
It appears the CM3L boards *may* suffer from this problem or something similar:
https://github.com/raspberrypi/firmware/issues/935
Can anyone verify?
For completeness, I must mention that we're using LAN9512s instead of the usual LAN9514s found on the Raspberry Pi 3 B boards, but I suspect that is not the issue here since it used to work.