I've never prewashed a negative. Do you simply run it under tap water or do you use your fingers at all to help remove the coating?
youi can just soak the film for a few mins, tap the tank to get the bubbles off and put it into the developer afterwards ..
does the stain look brownish when wet and when you hold it to the light ( when dry ) shimmers greenish yellowish ?
it might not be AH layer but
the dreaded fog i mentioned before due to the developer and your processing.
it happens and might because of using the wrong developer for certain types of film in your processor / processing
it is what happens if you use regular tmax developer ( instead of tmax rs developer ) and sheet film.
if soaking your film to remove the AH layer ( which is usually removed by the developer anyways) look up dichroic fog on the interwebs
and you can compare notes ... i got it years ago when tmax developer first came out, i was confused which developer was which
and called
kodak's pro hotline and the person there was confused too seeing i was advised to use the wrong developer ... it didn't happen
right away, i processed a bunch of film 1 shot to test and tweak my times and film and i was fine, then when i had a big job .. i was hosed.
when i called kodak they told me to throw away my film and shoot the job again. i had to go to a local photochemist ( paul krott who founded sprint )
and he told me to use farmer's reducer ( kodak's own packaged product ! ) kodak also told me the person who advised me didn't exist .. seems
i dreamt the whole thing up ... even her name ... thankfully scully and moulder helped me out afterwards ..