Do you use "compiled CSS into SWF" feature, i.e. "runtime CSS"?
I load the compiled CSS SWFs in from a ByteArray using module.load after the application has loaded and initialized. I'm using a rough copy of the code in StyleManagerImpl#loadStyleDeclarations2 to accomplish this.
Do you use single embedded font or multiple?
If multiple then you may be a victim of
"TextArea/TextConverter does not properly render runtime embedded fonts"
I have a little experience working with modules, but I'd rather bet there is a problem with loading embedded styles along your module, not RichText component or TLF itself.
This blog post by Gaurav Jain might be helpful to you as well to resolve your issue "StyleManager in Flex 4"