BackstopJS ignoring changes from white to light grey background

[pgn-vole]

It seems Backstop is not able to notice changes between grey and white background.
Here is a screenshot of the UI

Here are the test and reference snapshots:

I tried with other background (red, darker grey) and it is properly failing with those.

Here is the config beeing used:

{
    'id': 'component',
    'viewports': [
      {
        'name': 'desktop',
        'width': 1200,
        'height': 1240
      }
    ],
    'scenarios': [
      {
        'label': `${name}-examples-${theme}`,
        'url': url,
        'selectors': selectors,
        'hideSelectors': [],
        'removeSelectors': [],
        'delay': 500,
        'misMatchThreshold': 0.00
      }
    ],
    'paths': {
      'bitmaps_reference': 'visual-tests/screenshots/reference',
      'bitmaps_test': 'visual-tests/screenshots/test',
      'html_report': 'visual-tests/reports/html',
      'ci_report': 'visual-tests/reports/ci'
    },
    'openReport': false,
    'engine': 'chrome',
    'report': ['browser', 'CI'],
    'debug': false
  }

Please let me know if you need more information.