It sounds like different font families are applied to the header content and to the main body.

If, for example, Arial or Times is applied to Chinese text (and regular versions of the fonts do not define CJK glyphs) PD4ML scans its fallback tables to find a “similar” font, which is able to display a given text. If no suitable font found, it gives up with an output of question marks.

I guess it succeeds with a substitution font for header, but not for the main content. If you switch debug on, it should dump the font lookup attempts.