See ASSTR.ORG thread.
Yeah, it's a little annoyance. I've yet to locate the bug.
Clicking the green dot for the topic marks it as read as it should.
In addition to not automatically marking as read, once locked, it won't automatically scroll down to the last comment read/first unread comment.
I noticed this lack of automarking on another thread locked shortly after the change to https, but did not comment at the time (the green dot worked for me) - perhaps I should have?