(Update - Graf isn't out of business. Graf Switzerland, the main business, is still going strong.)
I contacted Graf Switzerland, and they said:
Can you send us a picture, please?
Without knowing here it separates off I can only say that the glue you have to use to repair it has to be an elastic one even when it is getting cold. If you use another one it is very likely that your heel is going to burst again. The best way is to as a shoemaker if he can stick it together.
Our heels are out of leather with a core out of hollow wood for the lightness.
Most of this separated heel are caused by to short screws. We advise our customers to use screws that are at least 25 mm (it's metric) long to prevent such cases.
I have done as they requested, and await their response. Of course, I want to use a glue (even if I decide to bolt it too) that can take heat molding temperatures (About 180, I think, for these boots), so it can safely be re-heat-molded. The layers separated cleanly, so maybe I don't need a "filler" glue.
Looking carefully, I think the layers that separated are both the same type of leather, at least on my boots. It is only the bottom-most layers, so the damage didn't reach to the wood core. I don't know why the other person thought they were plastic - maybe he was just unhappy with Graf.
I got these boots used. Interesting: With the bottom layer torn off, I can see that some of the attempts to fill past holes, including my own attepts with Shoe Goo, only went through the first layer. I need to rethink how to apply Shoe Goo in such a way that it is pushed deeper. I guess you can sometimes learn stuff by taking damaged things apart.
Unfortunately, I sometimes learn best by making mistakes...