BackstopJS ignoring changes from white to light grey background

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:
```javscript
{
'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.