Change "clean css" options in grunt min css

Question

Is there some kind of special syntax I need to use to override Clean CSS defaults when using Grunt CSS min?

Looking at the docs I can successfully get the "roundingPrecision" example working but trying to override other Clean CSS rules does not work for me.

I've given a config example below where I'm trying to prevent duplicate rules being removed (in this case I need a non-css variable fallback for browsers that do not support CSS variables). This is not working for me though.

Here is the CSS Min part of my Grunt config:

    cssmin: {
        options: {
            removeDuplicateRules: false,
            roundingPrecision: -1
        },
        prod: {
            files: [{
                expand: true,
                ext: '.min.css',
                src: ['<%= cssPath %>style.css'],
            }]
        },
    },

With the above config, this is what happens:

    .test {
       background: red;
       background: var(--colour-blue);
    }

Becomes

    .test{background:var(--colour-blue);}

However, the second background rule should not be removed.

I've noticed in a another StackOverflow question it seems like I may need to target some specific area of Clean CSS—Use clean-css compatibility options in grunt-contrib-cssmin—but I'm not sure how to do that for other properties like "removeDuplicateRules: false".

I'm guessing that I'm doing something fundamentally wrong. Any pointers would be helpful—thanks.

Update #2

As suggested I've tried to use the attempted "level" options as follows, without any success:

cssmin: {
  options: {
    level: {
      1: {
        all: false
      },
      2: {
        all: false
      }
    }
  },
  prod: {
    files: [{
      expand: true,
      ext: '.min.css',
      src: ['<%= cssPath %>style.css'],
    }]
  },
},