I had a similar effect some years ago with foma 120 film. I got tiny areas of blank film which are very hard to spot in the print.
The spots came from tiny paper fuzzies form the backing paper. I put my remaining films on ebay. Beside this, I always clear my camera from such spos every time I change the film. At the trip provisonal with the fingers. There are always some tiny paper pieces in the camera, even using Ilford film.