I promised to follow up. Allow me to sing the praises of Ilford.
I sent the bad film to Ilford for inspection. Within about two weeks, I got a detailed response via e-mail from a technician. She checked the batch number of the film and found no other complaints or anything unusual in the coating/finishing records. She found nothing unusual in my development or storage of the film. As such, her findings were inconclusive. She did admit that "latent image regression" was the one weakness of Pan F Plus, whereby films developed more than six months after exposure could potentially come out slightly faint -- but, she said, not nearly to the degree that I've experienced. Even so, she could only guess that it was an extremely rare case of this phenomenon, and, as she put it, "that you've been extremely unlucky." She and Ilford were very generous in replacing the film.
I've since had good luck, together and separately, with both Pan F Plus and Perceptol. In fact, I'm considering Perceptol (stock) as my developer of choice for Neopan 100 Acros, of which I have a lot on hand.
Thanks to Simon and to Ilford for their excellent customer service and standing behind their products.