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.