If the cause of the issue is what I think it is, then modifying the WordPress Patreon plugin could fix the issue. What I think is causing the problem is that the WordPress Patreon plugin is stripping the sso and sig query parameters that are sent with the Patreon login request. It could be worth contacting that plugin’s developers about the issue.
Before doing that, you should confirm that clicking the Login button on Discourse, for a user who is not currently logged into WordPress takes the user to the WordPress login page. If the user then selects the Patreon login option, they are logged into WordPress but not logged into Discourse. Note that if your Discourse site is set to be private, the above should happen if a user browses directly to your Discourse site. For that case, the user will not see a Login button on Discourse.