Null `metadata` property from `token` APIs

For APIs like /{address}/nft that return NFT information, it includes a metadata property. Can I get clarification on how the value of this property is set?

I have seen a few instances where different tokens from the same contract has this property
populated while others have it as null.

As an example, the first of these tokens (token_id: 68894) have the metadata property set while the other (token_id 23353) have it as null

    {
      "token_address": "0x6e4c6d9b0930073e958abd2aba516b885260b8ff",
      "token_id": "68894",
      "amount": "1",
      "owner_of": "0x87c066ed3142776f3df0b6bf329a72a7a672c7c5",
      "block_number": "12258081",
      "block_number_minted": "12258081",
      "contract_type": "ERC721",
      "token_uri": "https://api.influenceth.io/metadata/asteroids/68894",
      "metadata": "{\"name\":\"PN-35519\",\"image\":\"https://images.influenceth.io/v1/asteroids/68894/image.svg\",\"description\":\"A small, superior, Cis-type asteroid\",\"external_url\":\"https://game.influenceth.io/asteroids/68894\",\"background_color\":\"000000\",\"attributes\":[{\"trait_type\":\"Spectral Type\",\"value\":\"Cis\"},{\"trait_type\":\"Diameter (m)\",\"display_type\":\"number\",\"value\":3776},{\"trait_type\":\"Orbital Period (days)\",\"display_type\":\"number\",\"value\":1437},{\"trait_type\":\"Eccentricity\",\"display_type\":\"number\",\"value\":0.245},{\"trait_type\":\"Inclination (deg)\",\"display_type\":\"number\",\"value\":3.89},{\"trait_type\":\"Scanned\",\"value\":\"Yes\"},{\"trait_type\":\"Rarity\",\"value\":\"Superior\"},{\"trait_type\":\"Yield1\",\"display_type\":\"boost_percentage\",\"value\":3},{\"trait_type\":\"Metal1\",\"display_type\":\"boost_percentage\",\"value\":10},{\"trait_type\":\"Organic1\",\"display_type\":\"boost_percentage\",\"value\":10},{\"trait_type\":\"Can Mint Crew\",\"value\":\"Arvad Specialist\"}]}",
      "synced_at": null,
      "name": "Influence Asteroids",
      "symbol": "INFA"
    },
    {
      "token_address": "0x6e4c6d9b0930073e958abd2aba516b885260b8ff",
      "token_id": "23353",
      "amount": "1",
      "owner_of": "0x87c066ed3142776f3df0b6bf329a72a7a672c7c5",
      "block_number": "12258668",
      "block_number_minted": "12258668",
      "contract_type": "ERC721",
      "token_uri": "https://api.influenceth.io/metadata/asteroids/23353",
      "metadata": null,
      "synced_at": null,
      "name": "Influence Asteroids",
      "symbol": "INFA"
    },

For reference, the above is part of the result from this call:

curl -X 'GET' \
  'https://deep-index.moralis.io/api/v2/0x87c066ed3142776f3df0b6bf329a72a7a672c7c5/nft/0x6e4c6d9b0930073e958abd2aba516b885260b8ff?chain=eth&format=decimal' \
  -H 'accept: application/json' \
  -H 'X-API-Key: XXXXXXXXXXXXXXX'