If you have enabled time based scheduler "Adobe CQ DAM Expiry Notification" then by default this scheduler set with cron '0 0 0 * * ?' i.e scheduler runs daily at 12:00 am. so whenever the expiration date is reached once scheduler runs that asset will be deactivate/unpublished. if you have not enabled time based scheduler then that asset will be deactivated/unpublished right after expiration time reached. but in your case since you have not published the asset after updating expiry time, asset got deactivated right after prevous set expiration time(i.e Oct 1st) reached/when time based scheduler ran on publisher environment.