Clear. Well, it seems like some internal flag indicating this is a template gets lost. I have logged this as issue #3138 and we will look into it as soon as possible.
Thanks.
You need to set Document.IsTemplate = true; before saving. Aspose.Words resets this flag to false on open by default (see our API reference for Document.IsTemplate to learn why). If you set it to true, it will make sure Microsoft Word recognizes the document as a proper template and will fire FILLIN fields. It looks that Microsoft Word executes FILLIN fields for templates only, does not execute them for normal documents. I’m closing the issue, nothing to fix.