Ah thank you!
It looks like the solution is to add a CSS class generated based on the project name ("foo-web") to the <body> tag in the root HTML file (which is likely located here lib/#{your_app_name}/components/layouts/root.html.heex).
Though the number of lines of CSS happens to be the same (because the .important prefixer tailwind hack doesn’t do anything besides add a qualified CSS class), the contents do indeed differ.
My mistake was diffing the raw CSS at assets/css/app.css across branches - that file (for a newly created Phoenix app) just includes tailwind modules, and is 20 lines long. It and doesn’t actually represent the post processed CSS files (which I should have checked by downloading from dev tools 
).
The commit containing updated generator instructions is here: generator now explains to set the sandbox class on the body tag · phenixdigital/phoenix_storybook@be3295f · GitHub
