I had the same problem. Check out the answer at the bottom of this thread:
https://community.withairbnb.com/t5/Hosting/booking-window/td-p/270373
Where Shelly says "I think I found the solution to the Booking window delimma. Go to your calendar, select a couple of months worth of days that are just beyond 6 months out (or whatever booking window you have selected). The pop up box will show and your choices are Available, Blocked or Unavailable Until such and such Date Based upon your 6 month Booking Window. Select the Unavailable option. The dates will no longer show a $, nor will they have a line trough them (ie blocked). If you select a block of days inside you booking window, you will not get this Unavailable option. Good luck!!"
You will see availabiity options for each date past your booking window of:
1 - Available
2 - Blocked
3 - Unavailable until [DATE] Based on your X-month booking window.
Select option 3 to get back to using your normal automatic window of availability for whatever future dates you want to obey the window policy you've set.