I live in Puerto Plata and I surely would have heard of such a flight. I tried to travel from POP to Cancun about 6 months ago and was told there is a flight from SD to Cancun. Turns out the airline (Aero Mexico or Mexicana, cant remember which) could only guarantee the flight to Cancun, but they didn't know their schedule one week later to return. No joke.
I was told this by a travel agency and the airline itself.
I ended up flying from SD to Cancun via Panama City on Copa Airlines.