-JME-TrueTypeFont relies upon java.awt.Font for parsing the ttf files and sometimes Java's GlyphVector returns 0 paths for the shape contours. In this case, with mesh texts, no character will be displayed. I've also tried using Google's sfntly library for reading the font files and encountered the same issue.
-With some font files java.awt.Font reports a character as missing when it is actually present and replaces it with the default character. Again Google's sfntly library had the same issue with the same characters in the same ttf files.
-The true type specification discourages the use of overlapping contours, but it is allowed. The mesh triangulation does not work with overlapping constraint edges so these characters will not be displayed properly when using scaleable mesh characters.
Be sure to thoroughly test your fonts before including them in any final releases. If you have a font that you like and only a few rarely used characters are problematic you can use the TrueType3d.setInvalidCharacters(String text) method to ensure that those characters will instead be replaced by the default character. If the default character is problematic you can replace it with the TrueType3d.setDefaultCharacter(String text) method.