Override MAC address cannot get it back

There was problem when I tried to change the MAC address of my guest interface, it did take effect eventually.

The problem now is, it seems changed permanently and I just cannot get the MAC back to the initial state. Yes I tried even hard reset of the router and re-config it, still not working.

BTW, my router is Archer C7 v2, and running the latest stable version.

The override is configured in /etc/config/network, while it's supposed to be persistent, it won't survive a factory reset (firstboot).