[JimmXinu]

I have no idea what's in the source epub, let alone what's missing when split.

EpubSplit already copies over a number of different font file types. It should be possible to add more, if they can be identified:

Spoiler:

        ## always include font files for embedded fonts
        for key, value in six.iteritems(self.manifest_items):
            #print("manifest:%s %s"%(key,value))
            if key.startswith('i:') and value[1] in ('application/vnd.ms-opentype',
                                                     'application/x-font-ttf',
                                                     'application/x-font-truetype',
                                                     'application/font-sfnt'):
                self.add_linked_file(value[0])

Can you ScrambleEbook the source epub for me to look at?