Found a bug (in all versions of InDesign) where an IDML does not have the correct CustomGlyph string:

1. Create a text box and select the font "Zapfino".
2. Using the Glyphs panel, choose the character for the word "Zapfino" (which is GID 1059). Export to IDML. The IDML will open fine.
3. Select the character again and change the font to Times. The character will become "uhorngrave", which is GID 1059 in the Times font.
4. Export to IDML. Open the IDML and it will display as an invalid character.

The bug is because an IDML does not contain the Glyph ID and instead relies on the CustomGlyph string, but the name in the IDML is "Zapfino" when instead the info in the IDML should be "uhorngrave" for the Times font.