<style lang="sass" src="xxx.sass"></style> results in error after migrated to v15

[whitetrefoil]

Version

15.0.3

Reproduction link

https://github.com/whitetrefoil/vue-ts-arch/tree/2598ca63e3a0762b5d429a7a052261faabc97c0b

Steps to reproduce

npx gulp build

What is expected?

code built

What is actually happening?

ERROR in ./modules/hello/index.sass?vue&type=style&index=0&lang=sass (../node_modules/mini-css-extract-plugin/dist/loader.js!../node_modules/css-loader?minimize&safe&importLoaders=3!../node_modules/vue-loader/lib/loaders/stylePostLoader.js!../node_modules/postcss-loader/lib!../node_modules/resolve-url-loader?keepQuery!../node_modules/sass-loader/lib/loader.js??ref--9-4!../node_modules/sass-loader/lib/loader.js??ref--18-4!./modules/hello/index.sass?vue&type=style&index=0&lang=sass)
Module build failed: ModuleBuildError: Module build failed: 
@charset "UTF-8"
       ^
      Invalid CSS after ".hello {": expected "}", was "{"
      ......

---

IMPORTANT: This is sAss, not sCss.

This will cause error:

<style lang="sass" src="./index.sass"></style>

This is OK:

<style lang="sass">
.hello
  font-size: 150%
</style>

And here is webpack config:

      {
        test: /\.vue/,
        use : ['vue-loader'],
      },
      {
        test: /\.sass$/,
        use : [
          'vue-style-loader',
          'css-loader?sourceMap&importLoaders=3',
          'postcss-loader?sourceMap',
          'resolve-url-loader?sourceMap',
          {
            loader : 'sass-loader',
            options: {
              sourceMap     : true,
              indentedSyntax: true,
              outputStyle   : 'expanded',
              precision     : 8,
            },
          },
        ],
      },

[yyx990803]

Same issue with #1278 and fixed by 37329e1