The most likely cause is bad loading on the spiral, not the brief agitation lapse. If part of the film was touching or very close to an adjacent wrap on the reel, developer couldn’t circulate evenly there, causing localized underdevelopment marks that can appear to originate near the sprocket holes.

That also explains why only one roll was affected: each roll sits on its own reel, so a loading problem can happen to one reel while the other develops normally.

The top/bottom position is less important than whether that particular roll was seated correctly with even spacing all the way around. A missed agitation cycle can cause unevenness, but it would be more likely to affect overall consistency rather than produce repeated marks on just one film.

To avoid this:

• load carefully so the film feeds smoothly and doesn’t bind or overlap
• practice loading on scrap film, especially with plastic reels
• make sure the reel is clean and fully dry before loading
• confirm the film advances freely after loading
• keep agitation consistent and follow the film/developer instructions

So: one bad reel load is the best fit for the symptoms.