I had this happen when I programmed a route for a flight from YMML to YPDN:

YMML RWY16/KEPPA9 UH366 NATYA J19 BHI T25 TNK J30 TN VEGPU5A/LAPAR ILS28-X/LAPAR YPDN

As soon as I added the Approach, the amount of fuel remaining went from ~18k lbs to only a few thousand less than the total fiuel on board (I can't remember the exact fuel quantity I originally decided on for the flight).