Sorry Peter, hope you didn't think i was being condescending.
Wire enamels are somewhat more complex than simple thermosets and thermoplastics - I'm not a chemist so cannot explain why. One form of magnet wire insulation is called hot melts. The are typical pellets or powder that is heated until fluid and applied to the wire with excess being wiped off by the dies. Hot melts have limited end product use. Conventional enamels which use solvent to thin or control the viscosity come in a variety of materials that can meet a wide range of products.
While the following does not provide info about the actual varnish or enamel, it is a very good reference source of inf about magnet wire. If you put the following in a Google search you should get the document.
Hitachi Magnet Wire
Selection and Use Directions of Magnet Wires
I dd several Google searches using "magnet wire enamels" etc and I got a lot of information. in above Hitachi info it talks briefly about hydraulic degradation. Doesn't really explain how it happens but says it could.