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.