same issue here. bit desperate at this point searching for a solution.
wireguard used to work for me for long, when i was still on 6.8.x Unraid.
about 4 months ago, I upgraded to 6.12.13 i'm currently on, things still worked fine.
then in the same week, i made probably a dumb decision to also upgrade my router (from an old Optus/Sagemcom to a newer Optus/Sagemcom), server 192.168.0.X ip changed, all VPN tunnels then showed me 'UPnP: port forwarding not set'. all VPN stopped working for me.
i tried everything i could, deleted & re-added all tunnels numerous times, turned off the UPnP then on then off then on numerous times for testing, also changed router settings many many times. spent also some time switching back to my old router and actually i'm on my old router again now in bid of it would somehow work again. nope. felt like an idiot.
to me, it seems my router, or both routers, could not speak to my server well so the UPnP would not automatically be set. I recall that when i got it to work the first time, it took about an hour or so until the port forwarding rules showed up in my gateway themselves. but since 4 months ago, neither of the routers would set up the rules after even days of waiting ..
really hope someone could shine bit more light on this , if anyone has had the same issue and ever found a solution.. would be very much appreciated.