OK, it's not in every case, it's just when I had written comments on a photo and someone writes additional comments.
Say I have written a comment in March, on a photo by a friend named Alice (pseudo) :
And then Alice and I stop being "Friends" in Facebook. She doesn't allow anyone but her friends to access photos, so I shouldn't have access anymore. But today someone else writes a comment on that same photo and I receive a notification.
Let's click on the link to Alice's photo. Nice, I can view that old photo again! That I should be let in to see that photo and any additional comments is subject to discussion.
However, the big bug is that I can click on "Back to Album" and I get the complete album, which I certainly should not:
I don't know whether that's a common case or just a kind of local bug or exception...