There are a lot of buried layers, and of course unwinding this thing would be complete madness because it would never go back together. I'm sure you're right about what must have happened. It's just that the wire is so heavy, the enamel is not delicate at all, and there is such relatively small current flowing through the yoke that it seems unlikely. But in the absence of a better answer, this has to be the right one.
Photo shows the winding, along with the little green corrosion spot. Both the corrosion and black adhesive have since been removed and the wire beneath appears pristine.