Thanks @Siavash8 for helping us get this resolved!
As it turns out, if you're using FastCGI for your WordPress website it does not accept the authorization header which was causing the issue. Read this article for more explanation: https://wp-oauth.com/docs/common-issues/header-authorization-may-not-work-as-expected/15
Tribe has added a new checkbox called "Pass access token as query string". Once this is enabled, SSO will work as expected.