It sounds like Instagram is testing a new "Save Draft" feature.
Certain Instagrammers have reported seeing a dialogue box pop up offering to save their drafts, when hitting the Back button after composing a post.
Until now, backing out of a post would result in any content being lost.
While the update isn't massive, it would be a welcome addition for power 'Grammers and marketers managing multiple Instagram accounts.
Plus it might also help keep users on Instagram and away from Snapchat.