I'm having what might be the same problem.
At some point in the past random blocks showed up on my calendar and the availability window didn't seem to be working right, so I manually went in and opened the dates up out to the end of my calendar.
Now availability window is simply not working. I can change it to 6 or 9 months and it's blocked for a while but then becomes open again like this screenshot shows.
AirBNB's best response is this: Based on the investigation done by our Technical Team, it has been determined that you will not be able to make the changes to your availability window at this time. A Host can choose to make their calendar available for 3, 6, 9, 12 or 48 months in advance of today's date. If a Host chooses 3 months, a Guest can only book up to 3 months from now. Any custom availability set will not be overridden by changes to this setting.
After review, If a Host manually overrides a date as Available, then this action takes priority and the availability window setting is not effective anymore. You will need to manually mark the dates as not available based on x-month availability window option.
In short, since I manually opened dates up because of a calendar problem, my Availability Window feature is basically broken. That's what I hear them telling me, anyways, and they don't indicate that there's any way to fix it.