AEM | Adaptive Form | Form Data Model seems to not use data source configuration

I've set up a data source with authentication type OAuth 2.0. The connection seems fine and the node info is saved.

I've then used this data source in my Form Data Model.

When I try to test the service I get this error:

{
"errorMessage": "com.adobe.aemfd.dermis.authentication.exception.AuthenticationException: OAuthProblemException{error='invalid_request', description='Missing parameters: code redirect_uri', uri='null', state='null', scope='null', redirectUri='null', responseStatus=0, parameters={}}: OAuthProblemException{error='invalid_request', description='Missing parameters: code redirect_uri', uri='null', state='null', scope='null', redirectUri='null', responseStatus=0, parameters={}}"
}

It seems it's not using the data source values in the request?

Could I be missing some configuration?

Do any of you have the same issue? And do you know what it's caused by?

I'm using AEM 6.4.8.1 with the corresponding Forms release.

The request works in postman with same variables.