hideSelectors not working for reference, but for test (puppeteer)

I'm using hideSelectors on ".slick" slideshows as timing may otherwise cause differences.

For "test" everything works fine and as expected, but for "reference" the hideSelectors don't seem to work:
(The head should be hidden in both, but only test is correct)

This is my backstop.json:

```
{
"viewports": [
{
"name": "large",
"width": 1280,
"height": 920
},
{
"label": "medium",
"width": 1024,
"height": 768
},
{
"label": "small",
"width": 640,
"height": 920
}
],
"paths": {
"bitmaps_reference": "backstop_data/bitmaps_reference",
"bitmaps_test": "backstop_data/bitmaps_test",
"engine_scripts": "backstop_data/engine_scripts",
"html_report": "backstop_data/html_report",
"ci_report": "backstop_data/ci_report"
},
"report": [
"browser"
],
"engine": "puppeteer",
"engineOptions": {
"args": [
"--no-sandbox"
]
},
"asyncCaptureLimit": 5,
"asyncCompareLimit": 50,
"debug": false,
"debugWindow": false,
"misMatchThreshold": 2,
"scenarios": [
{
"label": "/wohnungsbau",
"url": "https://new.example.com/example",
"referenceUrl": "https://www.example.com/example",
"selectors": [
"document"
],
"hideSelectors": [
"iframe",
".slick",
".blazy",
".captcha",
"#cookiesjsr"
]
},
...
```

What am I doing wrong? I already deleted the entire backstop_data folder and run it again. I'm having the same issue on multiple different projects with similar configuration.