It is called mottling. A backing paper/emulsion interaction.
In the long past described by the industry as result of uncovered storage long time at most humid circumstances.
However recently a lot of these artefacts appear. One could argue that it is a feedback-loop internet phenomenon, But there a reports by people stating that they treatedd their films correctly, moreover not different as all years before.
But meanwhikle at least Harman (Ilford) have admitted that they got respective claims and indicated that indeed this is something new.
Basically a paper issue as such existed already fifteen years ago and I myself worked on alternatives.