Well contest creation and user submission are totally different.
I mean, contest creation announcement is made obviously when the contest is created.
The creation is made based on month day.
For the user submission, it's of course done when the user submit a media, so totally asynchronous.
However, for the start of voting phase, this is done based on album creation date.
Ie. it checks if the cron run date is between album creation date + number of days for media submission and album creation date + number of days for media submission + 1 day.
So I believed maybe it's there that an issue if the cron is not always run at the same time of the day.
Could you register for few days the time at which the MOTM cron runs ?