If you locate strange symbols in the fact value of your tagged note, you will also have the warning FactsWithNonDisplayableCharacters in your Validation report. If your Validation report contains the warning FactWithNonDisplayableCharacters, then it means that part of the text included in a specific hypertag has not been correctly transformed. The issue occurs during the PDF to xHTML conversion where some characters are recognized incorrectly, meaning that there is a graphical issue (Something we use to call as 'Unicode')
You are able confirmed this by checking the fact value found under the fact list. To solve this issue, you will need to assess how your visual component (PDF) has been created. Some features used in adesign software such as InDesign work perfectly fine for a PDF but for an xHTML format.
Contact Customer Support to get further details about the solution and what you need to relay to your graphic designers to fix the issue.