I bet you are right, it's in the perfect position to be the case rim. I won't worry about it unless I start to see issues feeding the last round, plus it probably won't get any worse as the case rim to bullet gap is only so large.
I am surprised that I've never seen this on any of my other mags, I also run MecGar's in my P226's but those use a black follower so perhaps these blue ones are just much softer.