I've got tailwindcss ^2.2.19 when I setup my project. I notice that jit didn't work. The output app.css size is 4 MB. I had to add --jit to my watch command:
npx: [
"tailwindcss",
"--input=css/app.css",
"--output=../priv/static/assets/app.css",
"--postcss",
"--watch",
"--jit",
cd: Path.expand("../assets", __DIR__)
],