The file date is only used when media are added during the course of a new library creation.
After that, the system’s date/clock is used. And it might be exactly this which has been set wrong when you added the affected movies.
In order to prevent this from happening again, make sure that the system time on your server is correct at all times. Verify that the ntp client is working properly.
Perform the Plex Dance with the affected items.
