Hi Katie -- Yes, that was a change in the April release. The reason for this change is that only the owner of a story (or an admin) can change its sharing level (private / org / everyone). This is how security in ArcGIS works for all items, not just story maps.
When the publishing options were available for co-authors, it made it seem like a co-author could open someone else's draft story and publish it to the org or everyone; however, this would simply give an error since the co-author doesn't have permissions to change the sharing.
If a story was already published then you may have been able to just publish changes without changing the sharing level of the story.
We are looking into enabling the option for co-authors to publish changes to an already published story in a future release, but we are thinking that authors should have to opt-in to allow that, and that will take us a little more time to think though and design. For now, all publishing controls are available only to the original author and admins.
Owen Evans
Lead Product Engineer | StoryMaps