I followed an answer-notification (shown in the top bar on SO) to discuss.area51.stackexchange.com. I was logged in automatically, I could respond to the answer with a comment, and edit my question to clarify.
Then I went to the main site, area51.stackexchange.com. I found I was not logged in; I had the
log in link in the top bar.
According to this answer I should have
(...) a single shared account between the two of them.
So how come I'm logged in on the discuss sub-site, but not on the main site?