source map sources filenames are generated incorrectly

[Goltergaul]

I have the following config:

{
    options: {
            plugins: [
            new (require('less-plugin-autoprefix'))({browsers: ["> 1%"]})
        ],
        paths: [
            'bower_components'
        ]
    },
    main: {
        options: {
            sourceMap: configuration.cssSourceMap, // enable when needed, consumes much time when compiling
            sourceMapURL: 'main_dist.min.css.map',
            sourceMapRootpath: '../../'
        },
        files: {
            'fileadmin/dist/main_dist.min.css': [
                'fileadmin/less/main.less',
                'fileadmin/dist/bower.css'
            ].concat(configuration.cssFiles)
        }
    }
}

This generates a sourceMap that begins with

{"version":3,"sources":["../../fileadmin/dist/bower.css","<input css 2>"],"names":[],"mappings":"AAoHA

What ist that <input css 2>? It should read ../../fileadmin/less/main.less or something like that?