Are flashes of light common after a surgery for retinal hole?

My mother has got a hole in her retina. The doctors performed a surgery by inserting a gas bubble in her eye and have asked her to maintain a face down position. After 4 months of the surgery, she still sees flashes of light in that eye, which are quite disturbing. The gas bubble has completely dissolved and the doctor says that the hole has healed completely. Are these flashes normal after surgery? Has she developed a retinal tear in her eye?