If I'm reading the makefile right, this looks like one of those devices which creates a separate wan_dev section in the network config.
If that's the case, it needs to be adjusted there.
There was some discussion a little while ago that LUCI was not properly handling this scenario, but I don't recall if or when it was fixed.
If guess it was after v18 though, maybe more like 18.06.2?
Context: MAC override does not work
and an OPEN Luci issue: https://github.com/openwrt/luci/issues/1100