I refuse to believe that agitation or chemistry issues would result in such precise, almost mathematically defined, symmetric pattern on both edges of the film.
I am a newbie when it comes to film development, but I am also an adult with an engineering background and the explanation above doesn't pass even on that level.
I admit I don't have an engineering degree, but while having one may sound impressive for a inexperienced newbie, that of course is irrelevant for just developing a film, where IMO just plain common sense rather then text book knowledge (let alone totally unrelated text book knowledge) is far more relevamt
I've shot and developed literally thousands of (b&W and color) films since the 70's up to the late 90's (when I switched to digital), as a budding amateur, a student at the Royal Academy of Arts in The Hague, and as a professional photographer
https://pbase.com/paul_k and without wanting to boast think I'm in a position to say your newbie assumption that the shown pattern isn't possible is totally incorrect
In your very likely inexperience as a admitted newbie, you very likely have very little, if any experience how certain types of agitation can indeed cause uneven development, or fixation
Let's just begin at the basics
It's very likely that as a newbie you slavishly follow the text book procedures when developing a film: pour in the developer, after the recommended time pour out the developer, pour in the stop bath, after the recommended time pour that out, and pour in the fixer. During all that time gently agitating the film by spinning the film reel with the little rod that came with the development tank (like with the below Paterson tank) in the center of the film reel

Paterson Universal developing tank
That procedure is just plain wrong
If you just pour in developer in the tank with a dry film, you very much risk creating tiny air bubbles on the film surface which can keep the developer from touching that area, and consequently small area's that get less development time (which will show as little white circles on the developed film)
Solution: pre soak your film before you pour in the developer with water of the same temperature as your developer for a couple of minutes, and agitate the film during that time
That agitation should not be faint heartedly: you want to get rid of any bubbles, and gently spinning the reel with the little rod won't be very effective for that.
Based on my decades long experience : take the tank in both hands, be sure to take a firm clap of the cover, and agitate the tank head over heels a few times. Then gently tap the tank on the table or sink or wherever you are developing your film(s) to make any bubbles possibly left on your film (you never know) let go from the film
Pour out the water, and replace it with the developer. During the development time, depending on how you want your negative (steep for more contrast having shot a low contrast scene, soft in case you shot a high contrast scene where you want to have a less contrast negative) shake it after shorter or longer intervals (of course using warmer or colder developer, and specific types of developer will also contribute to the end result)
Note that by just gently spinning the film reel you'll be basically moving it in one horizontal direction, left or right, risking oneven development (due to not completely having changed the developer in certain area's)
Next stop: pour out the developer, pour in water of the same temperature, and shake. Replace the water and repeat the same procedure a couple of times to be sure all developer is washed away out of any nook and cranny
I only used stop batch with the very first films I developed in the 70's. But after having read an article how the acid stop bath could risk a 'scare' reaction of the gelatin carrier of film (and having seen that actually happen, no internet back then to just pick up a story and with some self declared expertise spread it around) I stopped doing that (even if as theory will tell. stop will immediately stop the development process, while 'just' rinsing will allow it to continue due to possible residue developer on the film, based on my extensive experience : nonsense)
Poring in the fix after the extensive rinsing will basically have the same effect as the stop, but rather then stopping any possible developing, it will simple take the silver emulsion away which will leave any residue developer nothing the react with, basically having the same effect, but without risk of 'scaring' the emulsion
And with the fixer in the development tank when, like with the developer, just spinning the film reel with the rod will risk not changing 'saturated' fix enough to create a an evenly fixated ( i.e. without silver traces which will afterwards show as opaque stains on the developed, fixed and dried film)
So as described before, again shake the tank head over heels, and different from the developer, don't be too shy/scary doing so
The pattern shown on the film of the OP is based on my pretty extensive experience :-D of course possible with old fix, and too little, too much in one direction only movement (in this case too gently tipping over the development tank with the fixer) and yes, creative the traces on both sides (top and bottom of the developing reel) of the film
Firmly shaking the development tank with fixer will of course have no averse effect on the film: you're trying to get rid of any trace of undeveloped emulsion in any place, and firmly shaking will assure you the fixer will get in any nook and cranny
So based on my decades of real world, practical, deviating from the texbooks, experience (and the results to back that up) I dare say your based on unrelated knowledge statement however adult you are ("I'm right because I'm old' ? :-O ) is wrong