All custom front matter fields are getting removed during record extraction

Spent the last few hours with debugging.
Still don't understand what this row is for but it removes all the custom document meta datas from the records.

https://github.com/algolia/jekyll-algolia/blob/a79e8b4397b6fb6bce471d6fdaac8b29e422f6fa/lib/jekyll/algolia/extractor.rb#L36

What am I doing wrong?

Start in lib/jekyll/algolia/extractor.rb at the linked line and inspect how records are extracted. Reproduce the issue with a document containing custom front matter fields; done means those fields remain available in the extracted records.

Written by the indexing model from the issue text.