Testing against the endpoint looks like you have found a weird.
The API says “thumbnail_url” the documentation say “thumbnail_url” but it’s the profile picture being returned not a stream thumbnail.
Live/Not Live streamers returned by search the thumbnail_url is always the profile picture.
twitch api get 'search/channels?query=cohhcarnage'
{
"data": [
{
"broadcaster_language": "en",
"broadcaster_login": "cohhcarnage",
"display_name": "CohhCarnage",
"game_id": "490100",
"game_name": "LOST ARK",
"id": "26610234",
"is_live": true,
"started_at": "2022-02-16T12:24:34Z",
"tag_ids": [
"c2542d6d-cd10-4532-919b-3d19f30a768b",
"6ea6bca4-4712-4ab9-a906-e3336a9d8039",
"96b6073f-450d-4248-8ed4-988e28f3f759",
"54b544af-2d0e-4d17-a653-78e3f6fc1fc8",
"eaba0ad7-c4e1-4878-b37f-01308dbb65c8",
"1277e554-f8c9-4795-a475-27418d7bd111"
],
"thumbnail_url": "https://static-cdn.jtvnw.net/jtv_user_pictures/e3a58ce0-46ea-4558-8bb0-7a967c1d621c-profile_image-300x300.png",
"title": "[!DROPS] - !MERCH until 2/27! - Lost Ark (Ladon / Artillerist) - HORIZON: FORBIDDEN WEST SOON - !LAThoughts - YouTube.com/CohhDaily - !PC"
},
{
"broadcaster_language": "en",
"broadcaster_login": "cohhcarnage1",
"display_name": "CohhCarnage1",
"game_id": "0",
"game_name": "",
"id": "80339555",
"is_live": false,
"started_at": "",
"tag_ids": [],
"thumbnail_url": "https://static-cdn.jtvnw.net/user-default-pictures-uv/ead5c8b2-a4c9-4724-b1dd-9f00b46cbd3d-profile_image-300x300.png",
"title": ""
},
So something is wrong with search
I don’t know if this endpoint is supposed to return stream thumbnails or not (I never have a use for search myself)