If the spacing was increased you may have wound on a bit too far on the magazine insert, beyond the two arrows on the film, before inserting it into the A12 casing. That might lead to one less shot on the roll: did you get 12? Indeed it looks like maybe you just lost the last bit of the film emulsion cutting off the bottom of the last frame (which is the top of the last image). I don’t know anything about malfunctioning backs and what they produce but I agree it would be odd with a light leak to get only one frame affected. I’d say it’s just how you set up the magazine insert.