I am having the same login redirect problem that @markschmucker, @jimkleiber, and @marco.palumbo describe above, which I discovered a few weeks ago and knew had been working correctly sometime in May. Reading what you’ve said about it above leaves me convinced that it’s a regression in Discourse introduced in some update from May, because it hit all of us at the same time, we all had working SSO code, and we share no SSO code in common.
I’ve posted a bug report in hopes that we can get this fixed.