If on admin/config/development/performance/expire one sets it so that EXPIRE PROTOCOL does not include the base url, then the logic in the function boost_parse_url fails at

if (empty($parts['host']) || empty($parts['path'])) {
    return FALSE;
  }

as parse_url will never return a host name and no pages will ever expire after editing.

Appears to also be further issues with expire on a multi-site if $parts['host'] is commented out, then the filename is returned with double forward slashes,

cache/normal//tags/example-article_.html

which is understandable as the hostname should appear in between, but logging the filename EXPIRE PROTOCOL HOST is enabled shows no expiry of any pages because of a missing / e.g. it searches for

cache/normal/www.example.comtags/example-article_.html

and so also never expires an edited article.