Dear Ivica,
This is because of master styles, since the title placeholder is already Italic, you don't need to set it again, so delete this statement from your code.
portion.setFontItalic(true);
Since the master style is already Italic, when you set it italic again, it turns into non-italic, however, I will confirm it from technical team and let you know.
Many Thanks and Kind Regards,
Shakeel Faiz
Support Engineer
Aspose Tyumen Team