[Note: I’m not sure about the Bug category]
A custom network service[¹] opens a port on the system, but it’s not shown by the new cockpit server-manager.
It is, in the Network services page using the old NethGUI server-manager.
Using the firewall local rules page to display such services could be an option.
The Services page another option.
Also, not displaying them at all could be valid (you add it from the command line).
What is the expected way to have a list of open ports/services?