I was getting exactly what you show with my negatives, not all the time, but sometimes, I find that the reel can slide up wit inversion agitation, I changed to turning with the twiddle stick and all was fine, went back to inversion one day when I could not find the stick, and got the underdeveloped edge, so tried putting in the extra 100 ml and it has not happened again since, I would think it must be developer related, if it was film then there would be many posts on many forums about this, and yours is the only one, It will be interesting to here what Foma has to say, but in the meantime try the extra 100 ml and see how it goes,
Richard