having fiddled with image to exagerate the problem it looks to me that this due to either some contamination from the film spool or bubbles/froth on the film which means it was probably the the top film of the four poking out of the developer.
Reel could have slipped up the column and not moved back after inversions.
I don't think its due to flow of chemical over neg edge.
Either use more dev in tank or make sure you rap tank hard enough on base after inverions to make all reels slip down on column as far as possible.
On the other hand the dark line at the edge of it suggests it might be film spool contamination. Not sure about this.
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