description from ffnet doesn't have any HTML in it--just plain text--nor line breaks. replace_metadata is applied at that point, so there's no HTML to try and replace.
When FFF gives the
description to Calibre for book comments, Calibre wraps it in <div><p> ... </p></div>.
You could use replace_metadata to add your own wrapping HTML tags; I believe what tags are already there may affect what Calibre adds.