RSS Feed/News OAuth2 - Public flow - grant is always invalid

[ENXF NET]

I am trying to follow OAuth2 public flow and no matter what I do, I receive the following back:

JSON:

{
    "errors": [
        {
            "code": "invalid_grant",
            "message": "provided_code_verifier_does_not_match_code_challenge",
            "params": []
        }
    ]
}

Here's my code in generating the verify and challenge (it's in dart):
Code:

  // Generate a random code verifier
  String _generateCodeVerifier() {
    final random = Random.secure();
    final values =...

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