I had mottling issues (small dark specs in negs, white specs in positive images) starting in 2020. I was in touch with Ilford in late 2020 continuing into early 2021. My contact there was extremely nice. She sent a lot of replacement film, and we discussed the issue at length.
For me, storage issues and using expired film were not the problems. Problem was on their end.
In February 2021, she said they had altered their backing paper. She said in email:
"Films that are being generated now, have a lot tighter control on how the wrappers made – and lacquer levels have been increased, so that’s why I mention the films now will be more robust."
She said I should look for batches from 94 onward. Look at the code on the box:
"94AFN1C (or 94B or 94C etc), or 95A, or 96A etc to 99AFN1C. But after 99 – batches will show 01A, 02A, 03AFN1C etc. All more robust than pre 94."
I just bought some FP4 from B&H, and the code starts with 98. So if you're buying from a supplier who sells a lot of film, I imagine you'll be getting the FP4 with the improved backing paper now.
She also said they have identified a few things with actual film formulation that could make films more robust, but working on those things are longer term projects.