Stupid me... I got several Isolas myself... But at first reading I somehow got one of those Agfa folders in mind, just as those are more poular with Apugers than the Isolas. And when writing about the latter I totally forgot about my remark on those bellows.

Anyway, that makes a source less for that dust. But nevertheless with that retractable barrel you still have got an air exchange (pumping) that may bring in dust.

The Isolas are made from plastic (no Baekelite) and that can built up static and attract dust.
This dust may get onto the emulsion. (I find that issue of dust and static electricity tricky and practice showed me effects contrary to what expected.)

Try to clean that barrel from the outside as well as from the inside. And see if that helps.