I developed two rolls of c41 last night. One was an old roll of Kodak Pro Image 100 (obviously very expired) and the other was a new roll of Fuji Superia 800. Both were developed in the same tank at the same time. There is no visible staining or watermarks on the base or emulsion side of the negatives from either roll. I have developed a ton of color (including multiple rolls) and have never seen this before.
A long shot, but any chance the fuji got spashed while being loaded onto the reel, or while waiting to go into the tank? Looks like it developed, bleached and fixed in all other areas of the neg, and the other roll doesn't look too far off for an expired roll.
Maybe there was some bleach or fixer residue in the tank, which got flushed out by the developer onto one roll of the film? But hard to imagine such Jekyll/Hyde between two rolls in the same tank.
But I'm thinking it's more likely to be some sort of water or humidity damage well before you loaded the film in the reel.
Ya, I think the only (likely) logical explanation is that the roll was damaged prior to loading. Considering it's a new roll I'm going to guess it was some liquid. Probably water.
Looks like water damage to me as well. I have seen similar things when I was in the finishing business, the customer had dunked their film in water or the like and them brought it in to be processed.