It currently appears that any time an instructor publishes a module, the Canvas notification system generates an email which contains the page content for all pages in the module, even if the module is set to appear at a future date. This is a very easy way to accidentally release answer keys and other such restricted content, even if the content remains hidden in Canvas. Once the emails have been sent containing answer keys, there's no taking them back.
Under no circumstances should students be notified about changes to future modules. The date setting for the module should override any published/unpublished settings, and ideally, the notification emails should not contain the complete content from the Canvas page.