Gulp Autoprefixer - specified browser list ignored? -ms- prefixes missing

Question

Though there are similar posts on here, I couldn't find one explaining why Autoprefixer in Gulp seemingly ignores specified browsers in some cases.

Here's the relevant portion of my Gulpfile.js:

var sassOptions = {
  errLogToConsole: true,
  outputStyle: 'expanded'
};

gulp.task('sass', function(){
  return gulp.src('css/main.scss')
    .pipe(sass(sassOptions).on('error', sass.logError))
    .pipe(autoprefixer({
      browsers: ['last 2 versions','ie 9'],
      cascade: false
    }))
    .pipe(gulp.dest('css'))
});

I can tell that autoprefixer is running because -webkit- prefixes are applied to the final CSS file, like so:

-webkit-transform:translate(-50%,-50%)
transform:translate(-50%,-50%)

However, given the fact I've specified 'ie 9', shouldn't

-ms-transform:translate(-50%,-50%)

also be present here?

I've tried quite a few variations on the syntax, but can't seem to get anywhere! Any pointers here appreciated.