ActiveX controls saved in Office 2007 using the IPersistPropertyBag interface may encounter alterations to strings they save to the property bag. This occurs because the strings are persisted in the XML and must conform to XML syntax rules, which can force an alteration of the string that the control may not expect.

See the following page for the full details:
OCX Persistent Strings May Be Altered When Saved in Office 2007 Open Office XML Format