Now, i don't remember the exact sequence, but there is a clickable way to pick up a polaroid and not having it counted as found in the gallery, effectively losing it.
It happens if you pick up the polaroid and, instead of watching it, having it zoom in focus, you move on with the freeroam. I don't remember how you can do it, but you can. I noticed during the freeroam after the scene at the beach with the Redhead monstergirl.
So, mi guess is, the trigger that updates the gallery variables is when the polaroid is actively watched. The solution might be moving it to when the 'you found a polaroid' message pops up or directly when you click on the polaroid on the freeroam.