When the culture of the thread is the default one, the private font (e.g. "Shadow Brush") will be replaced with a substitute in the Html5 Viewer (e.g. "Verdana") as the viewer does not respect private fonts. This is normal and expected behavior. When exported to PDF the text is displayed with the correct font ("Shadow Brush") but the embedded font as claimed by the Font properties of the PDF document reader is the substitute font ("Verdana").
When the culture of the thread is changed, the private fonts are not respected and not embedded in PDF rendered document at all. The text does not appear.