You can find attached an XML report definition created with Telerik Reporting Q1 2013 v18.104.22.1681 and a screenshot from displaying the report in IE9. The issue is not reproducible with the mentioned version and the specified string passed as HtmlTextBox item value, so please double check if you have upgraded all references in your project correctly.
If you need further help, please send us the report definition and steps how to reproduce the issue.
Have you tried the new visualization options in Telerik Reporting Q1 2013? You can get them from your account.