Manganese dioxide should not form if the bleach is used properly. It is important that the bleach is acidic and then the permanganate will be reduced all the way to Mn+3.Permanganate bleach is unreliable and damages film, but is much safer for you to handle. And as it reduces, MnO2 precipitates onto the surface of the film, and also inside of the gelatin. Emulsion can peel off as well.
The only bleach that I have had trouble with is the copper one which caused severe softening and frilling of the emulsion