How fresh is your fixer? The reason I ask is that a while back I had an issue with what looked like pinholes with uneven edges on occasional frames of 120 film. I believe I traced it down to fixer. I used to mix 2L batch of
1:4 fixer and use that over several weeks/months as one-shot. When the fixer got older the spots started to appear on film. Running the fixer through a fine filter and eventually switching to always fresh-mixing it seems to cure the problem.