The PDF library used to export the content is quite limited and such cases cannot be properly handled.
Indeed, span elements with inline stylization cannot be exported and due to that I am unable to suggest you an easy approach to resolve this behavior.
What I can suggest is integrating a third-party or custom library to serve the PDF exporting functionality as described in the blog post
Another possible way is to implement a custom filter
that transfers the inline styles from the span element to the paragraph, although such logic could break different formatting in one paragraph.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.