config interface 'wg0'
option proto 'wireguard'
option private_key '62lKu9HsDVbyiPenApnc4sfmSYTHOVfFgL3pyB+cBL4='
list addresses '1x.x.x.xxx/32'
option dns '1xx.1xx.xxx.x' <-- gone after changing key in luci
So then I would advise you not to exclaim you've found a bug. I really donno how to better explain than others about Wireguard not having DNS. You merely route/allow the IPs of the proper servers thru Wireguard.
Otherwise, if your WG tunnel has a domain name, you have a chicken-or-the-egg issue where you need WG DNS before it can resolve the tunnel.
Perhaps you can look at threads that handle DNS leaks on VPN for more clarity.
My issue was the metric of the endpoint.
I will note that the disappearing setting does seem odd, though - since it exists.
I would fully document it for a bug report on LuCI.