I see the same behavior (at least it does not appear to affect any functionality). I noticed that it does not occur with older versions, which can be downloaded from
http://www.oldapps.com/foxit_reader.php. All v4 releases and v5.0.x and v5.1.x don't have this problem, but v5.3 and later do (note that 5.3.1 is mislabeled as 5.1.3 on that page). I think this would require a fix from Foxit.