Both golfers finished on 14 under par after their final round on Sunday, but heavy rain denied the pair the chance to battle it out for the top prize in the extra holes.
The event at the Sentosa Golf Club had already been reduced to 54 holes after most of Saturday's was washed out.
Pagunsan hit a round of 67 to force the play-off on Sunday, with a birdie coming at the 18th when he holed a 10-foot putt.
Fernandez-Castano, on the other hand, did have a three-shot lead going into the final day but a round of 72 wasn't enough to see him to victory.
The Spaniard admits he felt the pressure trying to defend his advantage over the rest of the field.
"It was a tough day out there - one of the toughest I have ever had," he said. "The heat was brutal and my game was not good.
"I was feeling the pressure because it has been a long time without a victory and I want this one desperately.
"I felt the pressure at the end. My game was just not there, the putter wasn't working and I made it tough for myself but at the very end to make that par on the last made my day.