[Bug]: OAuth Authorization Code request not working anymore after upgrading to 9.0.x

[datoml]

Describe the bug 🐞

After upgrading the Refit Library to Version 9.0.x my code for requesting an authentication token from AWS Cognito is not working anymore.
To confirm the issue I downgraded back to 8.0.0 and the request works.

The payload I am requesting is defined like this.

var payload = new OAuthQueryParameter {
    ClientId = $"{settings.ClientId}",
    Code = code,
    GrantType = "authorization_code",
    RedirectUri = settings.CallbackUri,
    Scope = settings.Scope
};

The interface like this:

    [Post("/oauth2/token")]
    [Headers("Content-Type", "application/x-www-form-urlencoded")]
    Task<OAuthResponse> PostAsync([Body(BodySerializationMethod.UrlEncoded)] OAuthQueryParameter parameters);

When I send the POST request the RESPONSE says that the method is not supported.
But based on my logging handler it should be fine.

Step to reproduce

Send a REQUEST to the AWS Cognito Endpoint to request an ACCESS KEY.
I know it will be hard to reproduce it for you.
Could you maybe give me an hint what changed between this two version?

Reproduction repository

https://github.com/reactiveui/refit

Expected behavior

The Request should work and a Response with the access key should be returned.

Screenshots 🖼️

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IDE

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Operating system

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Version

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Device

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Refit Version

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Additional information ℹ️

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