OAuth Server Flow issue Cannot get the whole State back the state is being altered/truncated.

While Trying to integrate Constant Contact with OAuth Server Flow there is an issue I am facing, I am unable to get the whole state back, the state is being altered/truncated.

For example . When I am requesting Constant Contact Service with the following url: URL:https://api.cc.email/v3/idfed/<client id> &<redirect_uri> & <response_type> & <scope> & <state>.

I am unable to get the whole state along with the auth code in my redirect uri, and also when I remove '&' symbol before the state in the URL mentioned above then I am able to get the whole state back properly.

So the following are the questions which I want a clarification/help on:

Why is it that the query parameters cannot be passed through state?