When I open a forum, I see all unread topics since my last visit marked as "unread". After I read one thread and go back by cliking the forum in the lower end of the thread, all former marked as "unread" topics are now marked "read".

That didn't happen since two weeks or so ago.

Was that intentional? Am I the only one and just too stupid to use the forums?

No, they've been going all crazy like that lately. As I understand it, there was a software update a few days ago and they've been subtly wonky ever since.
Admins are aware, but haven't yet been able to fix it.