In one of my interactions, I stated that the Mock API doesn't return any headers in its response. See below:

mockProvider.addInteraction({
        state: "",
        uponReceiving: `An invalid POST request to ${Config.config.SSO_TOKEN_PATH}. Returns 400 status and "invalid_grant_type" message`,
        withRequest: {
          method: "POST",
          path: "/v1/api/token",
          headers: {
            "Content-Type": "application/x-www-form-urlencoded; charset=UTF-8",
          },
          body: createFormBody(request),
        },
        willRespondWith: {
          status: 400,
          body: {
            error: "unsupported_grant_type",
            error_description:
              "Invalid grant type INCORRECT_GRANT_TYPE, expected authorization_code",
            error_uri: null,
          },
        },
      });

But for some reason, when the pact is generated, a header is added in the interaction in the API response

{
  "description": "An invalid POST request to /v1/api/token. Returns 400 status and \"invalid_grant_type\" message",
  "request": {
    "body": "grant_type=INCORRECT_GRANT_TYPE&client_id=34343433434&redirect_uri=REDIRECT_URL&code=AUTH_CODE&code_verifier=VERIFIER",
    "headers": {
      "Content-Type": "application/x-www-form-urlencoded; charset=UTF-8"
    },
    "method": "POST",
    "path": "/v1/api/token"
  },
  "response": {
    "body": {
      "error": "unsupported_grant_type",
      "error_description": "Invalid grant type INCORRECT_GRANT_TYPE, expected authorization_code",
      "error_uri": null
    },
    "headers": {
      "Content-Type": "application/json"
    },
    "status": 400
  }
}

Any ideas on how to stop this from happening

Why do you want this? Content-Type is strongly recommended by RFC 7231:

A sender that generates a message containing a payload body SHOULD generate a Content-Type header field in that message unless the intended media type of the enclosed representation is unknown to the sender