Looks like the film might not be feeding thru the camera properly so you're seeing the edge of the sprocket holes. Look at the negatives - are the exposed frames closer to one side of the strip of film than the other instead of being centered?
I think it's developer that's gushing through the sprocket holes, causing uneven development at the edges of the film. I HAVE seen that before, but with tank-developed B&W film that was agitated too vigorously with strong developer. I have not seen this on color film before, but it looks very similar. I am not sure how the lab is developing the film, but I suspect they have a quality control issue if you're seeing this regularly. C-41 process color negative film brand shoukld not affect this, but you're seeing it on Kodak film but not Fuji: that seems odd.
So it's not in the film itself, it's definitely on the film, on the emulsion side. You can kind of see it in the picture I attached. I don't think it's a camera issue, I've used different cameras. (These are all scans fwiw)
I do know it's a dip and dunk tank and they appear to be mostly happening at the top and bottom.
Now that I see this more closely, I am wondering if the camera back's light seals are absoluitely 100% light-proof...wondering if a very slight amount of light is leaking into the camera back,and is coming through the sprocket holes as the film is wound onto the takeup spool. This could be so,so little light leakage that the degree of exposure is at the level that it really barelty has any effect, except where light comes through sprocket holes.
Hmmm...I was thinking of much larger, more-diffused "surge marks", but these look different.
***RE-reading your post: wondering if the Developing lab's "ID flashing" machinery might be emitting a tiny bit of light as the rolls are loaded? At the start of every roll, an ID number is often "flashed" onto the roll. Wondering too if the light MIGHT be from static electricity flash from the film loader/opener mechanism....regardless, if this happens on multiple rolls from more than one camera, the issue is almost certainly the fault of the lab and its machinery or working processes.
The fact that the blemish is slightly offset from the sprocket holes...when film is wrapped onto itself, any flash or static that comes through the sprocket holes is offset, based on how many "wraps" the film has made onto itself, so the sprocket hole are literally, offset from one wrapped layer of film to the underneath layer of film.
I wondered if these were scans because when I've had film developed and scanned I've never noticed seeing edges of sprocket holes at all. The negative holder/carrier would allow light to pass just thru the frames of film so I thought maybe something was out of whack.
I don't see it in the photo but if it's on the developed film maybe check with the lab. Is it mostly at both ends of the roll, not at each end of cut strips? I'm wondering if something with the equipment is holding the film slightly tighter at both ends, that it's pressing slightly against something.
You could try asking on Film Photography Project's forum or their Flickr discussion group, or APUG (analog group) if anyone's seen this before.
Is it noticeable in the frames? on the actual photographs? I'd have to go get out some of my negatives to see if I notice it because I've never noticed it on any of my photos from the negatives. You shouldn't be seeing the sprocket holes at all on a finished print (unless someone's using a camera purposely that will give that effect).
I've had similar looking marks on some of my home-developed color film using Unicolor C-41 powder kit. For me it's when I squeegee off my negatives, stabilizer can remain in the sprocket holes and then form a row of spotting droplets just inside the row of the sprocket holes. I've started to take extra care when squeegeeing off my negatives, sometimes even running dampened gloved fingers down the spocket holes and then giving them a second pass of a water-dampened squeegee.
Sort of a balancing act between getting the spots off and not rinsing away the stabilizer.