The system interface has programmers working on it on a regular basis. They make changes in order to make it easy and intuitive for us to navigate. Administrators decided the system should close a thread after it's been inactive several months.
As you point out, recent threads are not closed, and so they should still be marked 'Open'. Yes, it certainly appears to be a bug. Your post is visible to administrators. We can assume they'll convey your message to the programming staff.