The irony is I’m in another thread w/ a very similar scenario minus the WG aspect.

You may be onto something more than you know.

I wouldn’t worry about that; I see the same for my Slate AX.

I think you’re close but I doubt it’s something directly set as a WG setting. I believe it to be the metric set to the routes.

I presume you can ssh into your GL device. Here’s an example of my routing table w/ a WG Client running. My Slate AX is behind a ‘main router’. I am using one ISP:

root@GL-AXT1800:~# ip route show
0.0.0.0/1 dev wgclient scope link
default via 192.168.1.1 dev eth0 proto static src 192.168.1.116 metric 10
10.14.0.0/16 dev wgclient proto kernel scope link src 10.14.0.2
3[redacted]7 via 192.168.1.1 dev eth0 proto static metric 10
128.0.0.0/1 dev wgclient scope link
192.168.1.0/24 dev eth0 proto static scope link metric 10
192.168.8.0/24 dev br-lan proto kernel scope link src 192.168.8.1
192.168.9.0/24 dev br-guest proto kernel scope link src 192.168.9.1 linkdown

In the case of Multi-WAN I would expect something more like:

# ip route show
default via 10.0.3.2 dev eth1  proto static  src 10.0.3.15  metric 10 
default via 10.0.4.2 dev eth2  proto static  src 10.0.4.15  metric 20

It’s the metric that dictates the priority interface. Can you post your route(s), please?