We see this very problem with PAN-f regularly and have brought it to ILFORDS attention on several occasions, to '0' reply. This problem can occur with all the ILFORD films but is most seen in the PAN-f. Because dr5 is enhancing 4~5x the negative counterpart, issues such as this are seen more clearly, but can be seen very well in film that is severely damaged, neg or dr5.
What we have determined is, that this is a storage or moisture issue. It mostly happens with the 120 film and we think the spots are paper related. Spotting can happen with 35 as well but they look different. If an un-sealed roll is put back into a cold-store, shot, fresh or old-film, the moisture will cause this spotting. PAN-f is the most susceptible to the problem. In most cases when a client of ours has this problem, this scenario occurred. This is what happened to your PAN-f.
We have been willing to give ILFORD our feedback over the years, good, bad and constructive, but they do not give dr5 the time of day.
regards - dr5