libcurl keeps a pool of its last few connections around after use to facilitate easy, convenient and completely transparent connection re-use for applications.
When doing HTTP requests Negotiate authenticated, the entire connection may become authenticated and not just the specific HTTP request which is otherwise how HTTP works, as Negotiate can basically use NTLM under the hood. curl was not adhering to this fact but would assume that such requests would also be authenticated per request.
The net effect is that libcurl may end up re-using an authenticated Negotiate connection and sending subsequent requests on it using new credentials, while the connection remains authenticated with a previous initial credentials setup.