I think I've figured out what is happening so this is more of a "is there a workaround" issue.

One of our use cases is converting an email to HTML (via the getHTMLText function on the Email class) and display it in a browser. With the latest update of rtf-to-html the HTML is now wrapped with a <pre style="white-space:pre-wrap">. While this does preserve line breaks, it also adds the default browser styles for pre tags:

     display: block;
     font-family: monospace;
     white-space: pre;
     margin: 1em 0;

Especially the font-family: monospace; completely changes how the mail is displayed since it overrides the font from the one we provide with whatever monospace font is available on the system.
I also can't just add a style for the pre tag since (as far I've seen) when using the getHTMLText function I have no way of knowing if the original mail is an RTF or a HTML mail. Adding this style would possibly change the font of a pre tag inside an HTML mail, where a monospace font is wanted. My current idea is looking for <pre style="white-space:pre-wrap"> and replace it with a span. Do you have any input on how to make this work?