I think Dr.5 is onto something - storage conditions.
It would be simple to see by taking some rolls of Pan-F+ in 35mm and 120, some protected from moisture, and others not, stick them in a freezer for a couple of years, and then test them.
As a general rule I do not freeze or cold store my film. I just keep it in the basement, protected from moisture, and I usually don't have more than 50 rolls of either 120 or 35mm at any given time, so that I rotate my inventory often. I haven't shot a lot of Ilford film in my life, it's been mostly Kodak, but the films that I have had problems with were those that had been frozen for a long time (by others). Some of it was Pan-F+, some of it was Agfa APX 25, and some Efke 100.
Good luck to you all in solving this. I recommend Ilford uses Simon Galley to collect samples from those that are having problems, maybe even exchange a few rolls of long time frozen film for fresh film - and run some tests... A logistical challenge and nightmare, but the remedy could be as simple as having defined recommendations of how to store the film.