I don't know if it's a bug or if the feature hasn't been implemented because the "Show Fediverse Address" option doesn't seem to do anything.
It would be good to see the communities' instances like this: [email protected]. I've joined several communities with the same name on different instances (e.g. [email protected], [email protected]) and it will be good to know which one I'm looking at.
As for users, perhaps have an option to display the instance of the user.
That should already be on the user profile under their name. Show fediverse address is for the communities screen to show the address of that community should you have multiple.
I can add a setting to show the full username instead of display name in.