span with background-color attribute exceed the document boundary

I have a problem with a Aspose.Pdf.Generator.Text containing html span with a background-color attribute. I've set the IsHtmlTagSupported to true;
This span doesn't work :
span style="background-color:#AFEEEE"

This one works :

span style="font-family:courier new,courier,monospace"

I'm using the 9.5 version of Aspose.PDF

Is this a bug or i'm doing something wrong?