The bug
In singleplayer without cheats enabled it is not possible to use the /publish command.
The pause menu does allow you to open the world to LAN, therefore /publish should be available to the player as well considering they share the same function.
Linked issues
is duplicated by 2
Attachments
Comments 8
I use publish to make the port 25565 so my friends can use my direct DNS to access it.
That way they don't need to re-enter a new IP:Port every time we want to play.
I have reworded your report a little bit because this is not limited to hardcore worlds. I hope you are fine with these changes.
Additionally as workaround you can use the "Open to LAN" button on the pause menu, though this does not allow you to specify a port.