If Plex doesn’t route out WAN1, it will by default route out WAN2. In doing this Plex will see it’s IP as the CGNAT IP and not the static IP of WAN1, This will make the server think there is no port forward available and route via their relay servers and not use the port forward. As it is a plex server it originates a connection to check what streaming capabilities it has (my understanding at least)
Same config as in post #5 http://forum.mikrotik.com/t/wireguard-wan-but-no-lan-access/168938/7
I don’t see how plex having it’s own route to WAN1 is making the WAN1 mangle route marked connection go out the wrong wan though