We too are using DiscourseConnect and experiencing the same issue.
We have it up and running since a few years now and everything worked smoothly. updated today to 3.5.0.beta8-dev [e91024a221]
Basically the callback from sso system to discourse url add https://discourse.domain.ext/login and we have the same screen as @markschmucker
We also noticed that clicking on the header logo we land on https://discourse.domain.ext/ and login is successful (just a click on a button is needed)
Seems like that in the previous version the session controller was behaving differently, likely understanding that the call was initiated by external sso and was handling it in the right way
I noticed that in the last month @zogstrip committed some changes that could be related (not 100% sure) to the misbehaving
By now we applied a workaround in the callback method that was adding /login to the discourse url and everything seems to work properly.
If I’m missing anything like some docs that was giving advice about a potentially disruptive change in this code portion just let me know.
Thanks yall for your support