Investigation in issue 662686 showed that downloaded fonts in macOS Sierra were not accessible. Following the out of process font loading path, it seemed strange that out of process loading for HanziPen is failing, however, the downloaded fonts seem to be collection font files, while our code 
  *out = CGFontCreateWithDataProvider(provider.get());
probably expects a non-collection, single font file.

It could also be failing because of the fact that HanziPen is rather large, about 69MB.

I suppose the easier solution in this case is really to widen the sandbox rules as proposed in issue 662686, but filing this in order to keep track of the issue.