I don't know why it wouldn't appear in the preview but can imagine that if
the paragraph format of the paragraph containig your Addressblock has either
double spacing turned on, or "space before" set to one line, you would get
double-spacing in the output since the Addressblock will result in multiple
paragraphs rather than multiple lines within a single paragraph. If so, the
solution would probably be to use a paragraph styl/format with no double
spacing or space before.