This issue is related with language=en-US parameter, with this parameter in url some API endpoints return not empty overview (and that's correct) but requests (also with language=en-US parameter) to similar endpoints like favorite-movies or watchlist-movies give us null overview which is incorrect.

To reproduce it:

1) Get movie details (request below returns not empty overview for given language - CORRECT)

GET https://api.themoviedb.org/3/movie/550?api_key={{api_key}}&language=en-US

The same happens on the list of top-rated movies (ID: 550 - Fight Club is high on this list so it's easy to check it here)

GET https://api.themoviedb.org/3/movie/top_rated?api_key={{api_key}}&language=en-US

2) Add movie to favorites

POST https://api.themoviedb.org/3/account/{{account_id}}/favorite?api_key={{api_key}}&session_id={{session_id}}

{
  "media_type": "movie",
  "media_id": 550,
  "favorite": true
}

3) Get list of favorites and search for movie with id 550 (overview will be empty - INCORRECT)

GET https://api.themoviedb.org/3/account/{account_id}/favorite/movies?page=1&language=en-US&api_key={api_key}&session_id={session_id}

4) Get list of favorites without language=en-US parameter (it gives us not empty overview - CORRECT)

GET https://api.themoviedb.org/3/account/{account_id}/favorite/movies?page=1&api_key={api_key}&session_id={session_id}