hmmm, maybe it’s the extension that’s not passing the referrer information in my case? I am using “Old Reddit Redirect”. The URL in that search result still shows www.reddit.com when I hover over the link, and when I open it and the browser extension changes the URL to old.reddit.com which is where I get the login prompt.

I guess other extensions might rewrite the link destination, but then that would require more access to the extension to read/write the contents of every site visited, which is too much for me.