After considerable searching I finally discovered the source of the contamination: looking into the material-supply chamber with the supply roll removed from the machine there's a bar across the bottom of the chamber to prevent the PVC from dangling out the bottom of the machine as it unrolls.
In the years I've been using the SD300 I have never cleaned there because the media door blocks the view of that area as the door opens.
When the door is fully opened, it totally blocks any view of that part of the machine. But when the door is closed, a full roll of material could easily come into contact with that area.
With the media door mostly-closed, I could see debris had accumulated in there over the years. And the bar was had traces of SolGL glue that had probably splashed there when I performed maintenance operations over the years. The glue stayed wet because SolGL doesn't dry up on its own, so naturally it rubbed onto PVC in the media chamber.
No doubt a professional serviceman would have cleaned this area, but I never called in a serviceman because I'd always tried to maintain the machine for myself and it hasn't ever needed professional repairs.
I thoroughly cleaned these hard-to-reach areas using copious volume of alcohol to dilute the SolGL glue and paper towels to soak it up. Sure enough, that stripe of glue stopped contaminating the tops of models.
A free, easy fix!