Wake on lan, weird mac in pulldown list

Using 21.02.0 but the issue was already present in 19.07.7 and maybe earlier.
I have installed etherwake, works ok, however my desktop pc is listed 3 times:

  • the real mac address
  • the fake mac address I told Windows to use so I can assign different hostnames / IP and avoid caching issues when dualbooting
  • another totally different mac, which I can't find under /tmp or /etc/config

How do I find out where this last mac address is coming from?