Apparently it was the Jetpack ‘prove your humanity’ math question that I encountered. As I was using a password manager it automatically filled in my login details and hit submit without answering the question. However it still logged me in without error. This will need investigating.
I don’t have this box checked and it’s only for the comment link anyway. My issue is when i have a custom link to my forum. I have got round this by using a plugin that lets me change the menu links depending on whether somebody is logged in or not.
The reason I want this is because I don’t want to force people to login just to read the forum. So if somebody is not logged in they go to the forum as normal, if they are logged in I change my menu link to include /session/sso.
This is the best solution for now I’m assuming but would be great to get this included as an option in the Discourse plugin so I don’t have to mess around with menu links. I would do it myself but I’m quite a PHP noob and don’t have a Wordpress dev setup currently.