I'm fairly sure it's the lab. I've run several rolls of Tri-X and Acros 100 through the same camera, managed in the same way. And I gave 2 rolls of E-6 to the lab in question, the roll of Velvia 50 this shot is from, as well as one of Provia 100. Both came back with issues: the Velvia has these marks through the roll like a paintbrush, and the Provia came back dirty with foreign matter stuck to it in small clumps. I'm either switching labs or just doing C41 at home instead.