Little square boxes? Incompatibility issue. Still not solved...?
Let me preface this by saying I only know two languages (English and American), so this reply is more of a font file perspective.
The rectangular outlines are placeholders for charactors that don't have a corresponding character in the referenced font file, Mandarin or not.
I've encountered a few of those funky specialty font files which didn't have a compete set of corresponding chars for a-z and A-Z.
What I suspect is happening is this one particular block has such characters that are not referenced in the font file, while the other blocks don't.
Perhaps there are some dialects involved too that aren't supported by your font file.
Perhaps a search for another more complete font file to assign to this block will remedy your OP.