The meaning of "visible" derives from vonuz "invisible." The prefix vo- means "un-," "opposite of."

"But" is a conjunction and "visible" is an adjective, so it comes down to context to determine what nuz should mean:

Monahven nuz nol gut "The Throat of the World is visible from afar."

Rok lost krin nuz mey "He was brave, but a fool."

Yes, it is an exception to one of our rules. 

What Paarthurnax said