For a short while I worked in the repair/rental part of a camera shop.
We’d get these old SLRs whose internal materials had deteriorated; especially foam rubber light seals and mirror dampers.
Some of them would turn to mush and/or flake off in little bits.
Maybe the Nikons mentioned have an internal materials problem that is shedding these little bits and pieces. Shame, if so.
Does any camera manufacturer have some purposely sticky stuff inside the image box to trap and hold debris? Directly under a self-cleaning sensor would seem to be a good place for it. Of course, the camera maker would have to make sure the the sticky stuff would have a long lifetime without becoming a contamination source in its own right. (3M lo-tack chemist, please pick up the nearest white courtesy phone.)
Lenses are another source of entry for environmental contaminants. Lots of lenses, especially zooms, work like bellows as their front barrels move to and fro. Some of the pro lenses have sealing of some sort, but it’s tricky when the internal volume of the lens changes as a result of either focusing or zooming.
The self cleaners do use a sticky trap but damp or gummy crud won't just shake off.
Can lenses be cleaned inside if dust or something gets inside one of them?