man cron
:
There are two ways how changes in crontables are checked. The first method is checking the modtime of a file. The second method is using the inotify support. Using of inotify is logged in the /var/log/cron log after the daemon is started. The inotify support checks for changes in all crontables and accesses the hard disk only when a change is detected.
Portanto, a única razão possível para ignorar as alterações parece ser que o mtime do arquivo é retrocedido.