One issue might be whether or not you have embedded the font into the document. If you do embed the font, when the document is displayed, it should use the embedded font. Otherwise, it will pick whatever font is on the system that closely matches the font specified in the document. Which might lead to different fonts being used, depending on the application.
If I am correct, this is not a LibHaru problem, rather an inconsistency/issue with the font selections by the application that opens the PDF.
Bill