I've just tested out one of the new Instant Apps to create an app using the Interactive Legend template. All works fine until I press the Publish button, at which point it hangs indefinitely (spinning blue circle). Unable to close the 'Publish' window or cancel. Google Chrome Version 91.0.4472.124 (Official Build) (64-bit).
Closing the browser window and accessing my ArcGIS Online Content I can see the app in the Content list, but opening it shows the default template (Interactive Legend Sample: Global Power Plants) rather than my customised app.
Is it just interactive legend or are you able to publish other instant apps? Also can you open the Chrome Developer tools and then hit the publish and see if there are any error messages in the console?