Space is big and Neutral Zones likely don't extend into infinity.
...Although the dialogue introducing the concept did specify that the RNZ was "between the planets Romulus and Remus and the rest of the galaxy", supposedly leaving no openings through which legitimate Romulan excursions to the galaxy could take place. Yet after the first few seasons of TNG, such excursions did take place. I think it was first in "Tin Man" that the presence of Romulan vessels in the same space with our heroes wasn't
automatically considered an act of war. Thereafter it became commonplace, in "The Next Phase" or "The Pegasus
" or basically every Romulan-related DS9 episode.