Autoprefixer cannot generate prefix for custom scrollbar

I use grunt-postcss to generate css prefix.

Configuration as follows:

processors: [
   require('autoprefixer') {
      browsers: [
        '> 0.04%'
      ]
    }
]

Sass as follows:

@mixin custom-scroll($thumb-bgcolor: $gray-background-color, $track-bgcolor: $white-background-color) {
  &::scrollbar {
    background-color: transparent;
    width: 5px;
    height: 5px;
  }

  &::scrollbar-thumb {
    background: $thumb-bgcolor;
    border-radius: 2px;
    min-height: 6px;
    width: 4px;
  }

  &::scrollbar-track {
    background: $track-bgcolor;
    border-radius: 2px;
  }
}

But, autoprefixer cannot generate prefix for scrollbar-xxx, such as -webkit-scrollbar, -webkit-scrollbar-thumb, -webkit-scrollbar-track. It is a bug or it is my wrong configuration?