Have you have enabled the ‘Create or Sync Discourse Users on Login’ option (found on the WP Discourse SSO Provider tab)? If so, an SSO payload is also sent when a user logs into your WordPress site. This payload also hooks into the wpdc_sso_params filter. Unfortunately, the results of this call to Discourse are not logged.
I’m going to make a small change to the WP Discourse code today so that the call made when ‘Create or Sync Discourse Users on Login’ is enabled does not happen if the request originates from Discourse.