Welcome!
See Forward and map port to container - #2 by stgraber for the differences between incus network forward and incus config device add ... proxy (proxy device).
I am not sure how incus network forward is implemented. If it is with nftables, then it would not appear under ss.
However, if you use a proxy device, then it will appear when you run ss. Because with proxy devices, Incus will launch an auxiliary process that will bind to the requests ports. Hence, it will be visible under ss or lsof.
In the following I launch a container, I setup a Web server in the container, and I expose that Web server on an IP address of the host. Initially, I try with port 80 on the host (already used, therefore failure), and then I try with port 8080 on the host (success).
$ incus launch images:ubuntu/24.04/cloud mycontainer
Launching mycontainer
$ incus exec mycontainer -- apt install -y nginx
...
$ incus config device add mycontainer myport80 proxy listen=tcp:0.0.0.0:80 connect=tcp:127.0.0.1:80
Error: Failed to start device "myport80": Error occurred when starting proxy device: Error: Failed to listen on 0.0.0.0:80: listen tcp 0.0.0.0:80: bind: address already in use
$ incus config device add mycontainer myport8080 proxy listen=tcp:0.0.0.0:8080 connect=tcp:127.0.0.1:80
Device myport8080 added to mycontainer
$ sudo lsof -i :8080
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
incusd 53378 1000000 3u IPv6 410518 0t0 TCP *:http-alt (LISTEN)
incusd 53378 1000000 7u IPv6 410518 0t0 TCP *:http-alt (LISTEN)
$
Finally, we test it.
$ curl 192.168.1.40:8080
<!DOCTYPE html>
<html>
<head>
<title>Welcome to nginx!</title>
<style>
html { color-scheme: light dark; }
body { width: 35em; margin: 0 auto;
font-family: Tahoma, Verdana, Arial, sans-serif; }
</style>
</head>
<body>
<h1>Welcome to nginx!</h1>
<p>If you see this page, the nginx web server is successfully installed and
working. Further configuration is required.</p>
<p>For online documentation and support please refer to
<a href="http://nginx.org/">nginx.org</a>.<br/>
Commercial support is available at
<a href="http://nginx.com/">nginx.com</a>.</p>
<p><em>Thank you for using nginx.</em></p>
</body>
</html>
$