Instagram is testing a new option to preview the placement of a post in your feed before you publish it
Instagram is currently testing a new option that allows you to preview what your posts will look like in your profile grid before you publish them, so you can make sure each post fits into your overall profile aesthetic.
Instagram post preview
Some users are now seeing a new "Show Preview" button during the post creation process. Click it and you'll see how the post will fit into the current format of your profile, including pinned posts.
This is a handy update that can help you decide what and when to post on Instagram so that everything harmonises visually.
For those who have this option, it now shows up for all images, videos, and carousels, but not yet for your Reels feed.