Scheduled Cron Jobs auto Deleted after few days

i am using CentOS Linux 7.1 with control panel webmin virtualmin and now after upgrade webmin i am facing a problem with my crontab and the problem is which cronjob i have made by me these all are deleted after few days . i mean i have create 3 cronjob which are running every midnight i have added them (19-MAR-2016) and now today when i check all cronjob which are added by me all are deleted but other system cronjobs still alive . i am adding my cronjob with user root . any idea regarding this?