Ok, i know this is a 6 month old topic but i had the same issue and i fix it (spending some time) before to find this:
https://tracker.moodle.org/browse/CONTRIB-4270
A possible fix is to change the line
$customdata = $DB->get_records('facetoface_session_data', array('sessionid' => $data->id), '', 'fieldid, data');
with:
$customdata = $DB->get_records('facetoface_session_data', array('sessionid' => $sessionid), '', 'fieldid, data');
And to be consistent fix also:
// Replace more meta data.
$msg = str_replace(get_string('placeholder:attendeeslink', 'facetoface'), $CFG->wwwroot . '/mod/facetoface/attendees.php?s=' . $data->id, $msg);
a couple of lines before