I keep getting 404's when trying to use the subdomain = true option in the Terraform config.

My configuration is as below:

resource "coder_app" "pgadmin" {
  agent_id = coder_agent.main.id
  name     = "PGAdmin"
  url      = "http://localhost:8800/"
  icon     = "https://simpleicons.org/icons/postgres.svg"
  subdomain = true
}

Which generates the subdomain https://pgadmin--main--workspacename--username.coder.domain/ in the workspace link.

However when I go to this link, the call does not get proxied down into the container. If I curl http://localhost:8800 in the container it works fine.

Can you please advise if I am doing something wrong?

EDIT
I have just tried using https://8800--main--workspacename--username.coder.domain/ and this works so the service is reachable but the coder_app subdomain stuff is not proxying correctly