If I remember correctly you are using admin siteaccess in cronjob, right? -s <admin_site_access_name>. It should clear cache automatically, can you check if there is same issue for normal content object with workflow?
" check if there is same issue for normal content object with workflow", I mean if the content cache is not cleared in front site access for user approval, the same issue should remain if you approve normal content(for instance, if you approve an article, the cache shouldn't be cleared as well in front site access).
I don't think running cronjob for 2 site access is the right way. If in <admin site access> site.ini, the front site acces is set to RelatedSiteAccessList, it will clear cache in front siteaccess. can you check if the front siteaccess is set like RelatedSiteAccessList=<FrontSiteAccessName> in setting/siteaccess/<AdminSiteAccessName>/site.ini.append.php?
