My opinion is the fact that it only happens on the last frame rules out a light leak in the camera. I am guessing the first thing you thought of was the loose roll so that probably isn't it. I don't know how labs load processing machines these day but some years ago I found a lab that did fog the last frame on every roll by something they had to do to load the roll. I would check with them even though you think they are beyond the error. It might be that they have to unroll the roll a certain amount in the process and they are unrolling it a bit too far allowing the frame nearest the outside to get a bit of light.