Usually the fixer. That is likely the anti-halation dye. I am not a fan of pre-wetting to remove dye or "soften" the emulsion for developing. Modern films have wetting agents incorporated in them. We see something similar with TMax films-takes lots of fixing or long wasing (with or withoiut hupo eliminator) to get dye out. I would just was the films a bit longer in the future. Best, DFS.