What happened to the scar??

As a boy, Gene Kelly had a bicycle accident that left a scar above the left end of his mouth. It *invariably* shows up in his close-ups--or so I had always thought. About 21 minutes into the movie the camera goes back and forth from Kathryn Grayson singing, to Gene Kelly sitting at a soda fountain. The scar in most of the shots in that sequence doesn't appear at all, and only barely in others. If the scar could be covered by make-up or removed by touch-up, why wasn't it done consistently, instead of only in these particular shots, and in this particular movie?