How are you developing the negs? Drum, trays, hangers?
I was having similar problems with FP4 in expert drums -- solved it by using a fix without hardened (was using Kodak Rapid Fix, so I left out Part B). My best guess on why is that the antihalation layer was slow to be removed since the back of the film touches the drum. The hardener then hardened or reacted with the remaining antihalation layer.
Just a guess -- I am just happy that the fixer w/o hardener solved the problem...whatever the problem actually was.