Using grunt to compile sass, concatenate and minify, I discovered z-index entries were being set to 1.

This is a known issue with cssnano. See and

In the readme for cssnano, it says:
Note that cssnano enables aggressive optimisations by default, which might not always be what you want. Set `` to `true` if you want to disable this. In future versions, only safe options will be enabled by default, starting from [version 4][v4].

This is how you set to true if you are using grunt.

postcss: {
            options: {
              map: true, // inline sourcemaps
              processors: [
                require('pixrem')(), // add fallbacks for rem units
                require('autoprefixer')(), // add vendor prefixes
                require('cssnano')({ "safe": true }) // minify the result
            dist: {
              src: 'css/style.css',
              dest: 'style.css'

If I just saved you an hour of mucking around, please let me know in the comments!