Thanks for your feedback on this, the forum thread here and firmware update instructions in the manual have been updated to suggest using a different port.
The long explanation is:
In recovery, eth0 is configured is the “LAN” (192.168.10.1/24) interface behind the bridge by default and the set-wan script is not sophisticated enough to properly bring it down. It’s something I plan to rewrite soon to fix this and support a few other wan types (e.g cellular)
When eth0 must be used, you can do:
ifdown lan
uci delete network.lan # just to ensure OpenWrt doesn't try to start it again
set-wan eth0