Looks like the film hasn't been fully immersed in the developer and fixer, in some tanks the spiral rises up the column and then isn't fully submerged. Paterson and Jobo use locking rings to ensure this doesn't happen, often these don't get used.
It looks like something like that has occurred, you need to check your tank.
Ian
Ian, I think you're correct that it's the tank but I can't deduce the why.
I've been stand developing using a daylight tank with hangers. After much practice and waste effort, this is a process that now works fine. But the daylight tank will not work fine for normal development because agitating causes surge marks through the hangers.
So I stand developed using this Fedco sheet-film tank which DOES use a sort of spiral, or what I think of as a spiral. The end goal is to get comfortable enough using this tank for normal development (a la agitation). It holds up to 12 sheets of film and is adjustable up to 4x5.
I've attached pictures to illustrate.
Prior to souping the film, I ran a few tests to determine 1) how much solution is needed to fill the tank to the top so that it covers the entire spiral but doesn't spill over (Answer: 1.5L); and 2) in which direction to place the film (Answer: emulsion side out; emulsion side in gives uneven development around the edges). I practiced loading and unloading several times in the light.
So the current problem is that I filled the tank with 1.5L of developer and 1.5L of fixer. So I don't understand how 1/4 of the frame could not have been submersed in one or both. Maybe I'm going crazy? It's such a clean line demarcating the problem area that this result would make sense if I had only added, say, 1L of developer instead of 1.5L.
But I did not! Unless I am in fact going mad and can't measure.
Spiral with one film loaded, next to tank.
Film loads into spiral from one end only. This is the slot the film was loaded into during this development.
Spiral with film loaded into tank, from above; with lid.
Tank loaded and ready for solution.