In order to be able to save drafts of a post or page that is currently published we have the Live Drafts plugin installed. One of our customers reported an issue with this Plugin where they did a preview of the page and when they returned the Save Draft button was no longer available.
Because this plugin has not been updated in over 2 years we have created our own version of this plugin so we can make improvements ourselves.
The issue has now been resolved, you can now use the Preview button and still have access to the Save Drafts button. Don’t forget that once you save a draft version you should always work on the draft page until you publish it or decide to abandon the changes.