Why would most FontAwesome icons work but not one?

[Mike Hearn]

I'm using FontAwesomeFX in my app (thanks for that guys!) and it works
brilliantly .... except for one icon (THUMB_TACK), that is being set as a
graphic in a listview. It works fine on MacOS but renders only a box on
Windows. The icon shows fine in the fontawesome website.

The only explanation for this I can think of is that something is going
wrong deep inside the Windows specific text rendering pipeline. Other FA
icons work, so, I'm puzzled as to why that one might be different.

Does this sort of thing ring any bells? I'm unsure where to begin the
debugging process.