Same here.
One possible solution that seems straightforward: Is there a way to pass custom query parameters to the Discourse site’s /session/sso endpoint to allow additional data to be added to the payload passed through the SSO system? If so, the token could be passed through successfully using something like:
https://mydiscoursesite.mydomain.com/session/sso?token=abc123
(I should say I tried the above, but the extra parameter was not incorporated into the SSO payload.)