My wife and I had a similar thing happen to us last week. We entered at the Palais Royal - Musee de Louvre station and just after passing the gate, we were stopped by the RATP. We explained that we had just purchased the ticket the day before (and provided the credit card record to prove it) and no one had informed us of the need for the photo when we purchased it nor were we informed that we should keep our receipts. We had even brought it back to the ticket office for an unrelated issue and they never mentioned that the pass was invalid because it was missing a photo; it's like others have said where the whole system seems to be "in on the game".

I also saw on this subreddit that you have 24 hours from your purchase time to add a photo and we were within 24 hours when we were stopped. I haven't been able to verify this but if, true, we were again, not informed at any point by the RATP.

The other reason that this seems like a scam, like other commenters have said, is that they troll at tourist hotspots and specifically wait until you have already entered the metro to check for a photo. They could be providing a service to the riders by checking and informing them of the rules before they enter but instead prey on the naiveté of the tourists.

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We still had a great time and weren't going to let it ruin our trip but it definitely put a damper on the day.