I'm having this problem too. I had it twice this week, also right near the mute button. On the first phone, I thought maybe the frame was bent or something. Now that it's happened again on a second phone, I'm frustrated.
I tested both phones to see if the frames were possibly bent. Basically, I took the screen off and laid the phone face down. The phones laid flat and evenly so I'm sure that this wasn't a bendgate problem.
I like the suggestion that Apple should put a clip somewhere by the mute button, but that doesn't help the current situation. Has anyone else had this light leakage problem after replacing an iphone 6 screen? What's a good work-around?
My screens are higher quality from a supplier that I trust.