RequestClientCredentialsTokenAsync and ClientAssertion Encoded

Question

I have a strange behavior with this method and the way the server receives the information, can't figure out why it happens.

So I have this simple method that calls the RequestClientCredentialsTokenAsync

private async Task<TokenResponse> Token()
{
    var requestBody = new ClientCredentialsTokenRequest()
    {
        ClientId = "superClientId",
        RequestUri = new Uri("http://something"),
        GrantType = OpenIdConnectGrantTypes.ClientCredentials,
        ClientCredentialStyle = ClientCredentialStyle.PostBody,
        ClientAssertion = new ClientAssertion
            {
                 Type = OidcConstants.ClientAssertionTypes.JwtBearer,
                 Value = "value_created_in_another_place" 
            }
        };
    using var httpClient = _httpClientFactory.CreateClient(Options.DefaultName);
    return await httpClient.RequestClientCredentialsTokenAsync(requestBody);
}

So this function uses the constant urn:ietf:params:oauth:client-assertion-type:jwt-bearer, and nothing special is going on here.

On the server I got this for handle the requests:

public async Task<IActionResult> Post()
{
    using var reader = new StreamReader(Request.Body);
    var tokenRequestDto = new TokenRequestDto();

    var data = await reader.ReadToEndAsync();
    if (string.IsNullOrWhiteSpace(data))
    {
        return BadRequest();
    }
    //Other stuff to do...
}

The problem here it's that the constant for some reason gets encoded, so instead of the original string, I get something like this:

grant_type=client_credentials &client_id=superClientId &client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer &client_assertion=...

Already tried to change the StreamReader and passed some encodings, Default, ASCII, UTF8 but in the end I got the same thing.

Just trying to understand why this happens. Anyone has some idea/information?

Thanks