I am publishing short form, too, and I am facing similar issues. I did some experiments yesterday: The continuous stream works on smartphones if you "Toggle Medium beta" (available in settings). When opening the publication, the short form is displayed in full.
Nevertheless, on laptops, just a preview and a "Read More" button appears. I've contacted Medium support on this issue. Apparently, it all comes down to the paywall: If you meter the story, a "Read More" button appears. If you opt not to meter it, it appears in whole on profile streams.
I will experiment with this further and give an update if I know more.