This is a known issue, and has happened to several hosts, including me. I blocked off my calendar when I accepted a six-month visitor, but three times random weeks during that six-month period mysteriously became available.
Airbnb apologizes, but it sounds hasn't seemed to be able to solve it yet. The best I can recommend is to be vigilant and check your calendar occasionally. The good news is, you can simply and politely decline any requests that come in for times when you are already booked. (This is another reason why I don't use Instant Book--calendar bugs can be much more troublesome for those who don't book exclusively through Airbnb.)
You can also snooze your listing so it won't be available on search for a period of time when you are not seeking guests through Airbnb, but as you point out, it's not what you need right now.