The Treaty of Algeron (no clever mice there!) appears to coincide with the loss of contact with Romulans, at least in the usual coarse terms of "barely a whisper for fifty years" or "gave them the tactical advantage for sixty years" that we hear used. We're left to wonder whether the victors of the relevant engagement would choose to isolate themselves behind the Neutral Zone they so openly loathe and violate later on.
Perhaps the Romulans had to give up something so significant in return for the cloak thing that they were forced to put on ice all their plans of "interaction" (conquest, sabotage, espionage) with the UFP for fifty years. Say, they
might have given up cloaking for the same period of time, too!
The Feds seem to feel obligated by the treaty even after the Romulans demonstrate cloaking in "The Neutral Zone", admittedly. But they're the good guys. And they might not want to issue a challenge when Romulans are using cloaks on their side of the RNZ, or when both the Romulan and Starfleet ships involved are violating the RNZ.