Wow! that's quite a defect. I imagine it's the point where the plastic lifts off the buck just before the vac is applied. After the vacuum pulls the plastic in on the buck it kind of stretches or pulls at that point probably because there is a temperature difference between the plastic touching the buck on top and the plastic that hasn't lost as much heat. I can think of a lot of reasons for this. An extremely cold buck, a sticky buck (more talcum powder to allow the plastic to move on the surface), Thicker plastic that won't lose it's heat so fast as it touches the buck or quicker vacuum application. Interesting problem to solve. I hope you get it worked out with the vendor Sarah.