I am referring to this page: https://developer.themoviedb.org/reference/movie-details

I formed a request to this endpoint and noticed that there is an undocumented field in the root object (not the field inside the production_companies object):

"origin_country": [
    "US"
],

This field appears to exist for the TV show details endpoint documented at: https://developer.themoviedb.org/reference/tv-series-details

Hope this helps.

P.S. I couldn't find a way to directly make a pull request to suggest this change. This discussion is the best way I knew how, to report this.