I'll describe this like a bug report because it seems like a bug.
I was logged into SE and able to move freely from site to site (among the sites I have already joined) -- EXCEPT Super User. To get to Super User, I had to log in again (same email address, same userid, same password).
I do not remember ever having had this problem previously.
Another anomaly: Super User was then automatically added to my list of communities.
Is it supposed to work this way?