When a style doesn't have explicitly defined font, the font from the default character properties (\defchp) is used when present. Instead, the font used for the style should be the default font for the document, defined with \deffN tag.
The same issue occur when the doesn't contain \defchp at all. In this case, the font for the style is not imported, but instead the defined with  \deffN tag should be used.

The construction is not common, and MS Word for example doesn't produce such documents.