I suspect that the Part B might be old or bad. What color was the PMK after you mixed Part A with Part B? If it was a red color that would be an indication that everything was not as it should be. I have seen what you described but only once. The film you are using is supposed to be a rebadged FP4. FP4 in my experience does not stain like other films like say HP5+. Hutchins has stated that the after fixer bath in spent developer is not really needed. You might re-run the test again with new Part B and see if it happens again. You can make new Part B with 850 ml of h2o and then pour in sodium metaborate until the volume raises to 1000 ml.