From agent/ports_supported.go

getListeningPorts() currently only detects processes listening on IPv4 addresses.

As an aside, I discovered this because I'm running a Go process in a workspace which I instructed to listen on 0.0.0.0:8080 but Go binds to IPv6 where possible, so it got bound to :::8080 and subsequently was not listed in the list of listening ports, counter to my expectations.

From https://pkg.go.dev/net#Listen:

For TCP networks, if the host in the address parameter is empty or a literal unspecified IP address, Listen listens on all available unicast and anycast IP addresses of the local system. To only use IPv4, use network "tcp4".