Fixed. I've had this problem with vanishing images before. The only fix was a re-make of the file under a different name.

This is why it's weird;

1. Simply re-uploaded file - no fix
2. Remake file without rename - no fix (even with different palettes)
3. Renamed file to [original-name]2.gif - no fix
4. Remade and renamed to [random-name].gif - fixed