The vertical, rectangular banding seems to mimic the internal sprocket marks that connect JOBO tanks, such as the 1502 and 1503 that I used.
Since they were new, they were also pretty stiff. I hope it wasn't the case that I didn't connect the tanks fully, leading to light leaks that made that rectangular banding pattern, but the more I think about it, the more that might explain a lot of the leaks.
I thought I triple checked how secure the tanks were attached to each other prior to loading the film, but the connectors seem to require a bit of rough handling, and do not click together easily. Maybe I just didn't connect them as well as I thought.
If there are any other JOBO users that may have done the same error, I was hoping they would chime in to see if similar banding has happened from not perfectly sealed extension tank to regular tank.
Will shoot a test roll tomorrow and dev similarly.