The game ended with a 4-3 win for Binh Duong.
After watching a video of the game, the Vietnam Football Federation said the foul had indeed been committed inside the box and should have resulted in a penalty.
De’s decision was a "serious mistake that affected the result of the match," it said.
This is the second time that Nam Dinh have lost a penalty due to a referee error. In their match against Viettel on March 19 too Konan was fouled in the box, and again the referee, Nguyen Viet Duan, did not point to the spot.
They lost 1-2.
They are seventh in the table with 12 points.
Earlier this month Le Duc Canh was suspended for two games after several poor decisions in the match between SHB Da Nang and Hanoi FC.