mm or cm Units are converted to px

Did you add mm, cm etc, as units to the style manager inputs, these are not built in

As stated above it's what the browser engine does, usually this is the fix, but I've never used it with units that aren't built in:

window.editor = grapesjs.init({
   // ...
   styleManager: {
      // ...
      avoidComputed: ['width', 'height', 'padding', 'margin'],
      // ...
   }
   // ...
});

I think that's just what the browser engine does in general. I'm no expert, but I think that output is just the final info interpreted by the engine.