It's already part of the collection.

For some reason the API doesn't return this info.. i.e. "belongs_to_collection": null,

See response below:

{
  "adult": false,
  "backdrop_path": "/6ELJEzQJ3Y45HczvreC3dg0GV5R.jpg",
  "belongs_to_collection": null,
  "budget": 200000000,
  "genres": [
    {
      "id": 28,
      "name": "Action"
    },
    {
      "id": 12,
      "name": "Adventure"
    },
    {
      "id": 14,
      "name": "Fantasy"
    },
    {
      "id": 878,
      "name": "Science Fiction"
    }
  ],
  "homepage": "https://marvel.com/movies/movie/224/black_panther",
  "id": 284054,
  "imdb_id": "tt1825683",
  "original_language": "en",
  "original_title": "Black Panther",
  "overview": "King T'Challa returns home from America to the reclusive, technologically advanced African nation of Wakanda to serve as his country's new leader. However, T'Challa soon finds that he is challenged for the throne by factions within his own country as well as without. Using powers reserved to Wakandan kings, T'Challa assumes the Black Panther mantel to join with girlfriend Nakia, the queen-mother, his princess-kid sister, members of the Dora Milaje (the Wakandan 'special forces') and an American secret agent, to prevent Wakanda from being dragged into a world war.",
  "popularity": 95.932,
  "poster_path": "/uxzzxijgPIY7slzFvMotPv8wjKA.jpg",
  "production_companies": [
    {
      "id": 420,
      "logo_path": "/hUzeosd33nzE5MCNsZxCGEKTXaQ.png",
      "name": "Marvel Studios",
      "origin_country": "US"
    },
    {
      "id": 2,
      "logo_path": "/wdrCwmRnLFJhEoH8GSfymY85KHT.png",
      "name": "Walt Disney Pictures",
      "origin_country": "US"
    }
  ],
  "production_countries": [
    {
      "iso_3166_1": "US",
      "name": "United States of America"
    }
  ],
  "release_date": "2018-02-13",
  "revenue": 1346739107,
  "runtime": 134,
  "spoken_languages": [
    {
      "english_name": "English",
      "iso_639_1": "en",
      "name": "English"
    },
    {
      "english_name": "Korean",
      "iso_639_1": "ko",
      "name": "한국어/조선말"
    },
    {
      "english_name": "Swahili",
      "iso_639_1": "sw",
      "name": "Kiswahili"
    },
    {
      "english_name": "Xhosa",
      "iso_639_1": "xh",
      "name": ""
    }
  ],
  "status": "Released",
  "tagline": "Long live the king.",
  "title": "Black Panther",
  "video": false,
  "vote_average": 7.4,
  "vote_count": 16525
}

The response for Black Panther 2 doesn't have this issue and it shows up as being part of the collection..

There was a glitch with this one, where the film showed on the collection page, but the collection did not show up on the film's page. I had to remove the film from the collection page and re-add it to fix that glitch. The change should update to the API once the cache expires.