@Rachel243....yes Rachel, that's what happens! It's being going on for more than a year now and there seems to be no light at the end of the tunnel towards getting this 'glitch' fixed. The issue does not appear to be a big enough one for Airbnb to take any action on.
As far as I know Rachel the best way around this problem is to set your booking calendar to 'All Future Dates' and set a start and end date for dates that you want to block. What is happening at the moment, you may have set a booking window of 6 months and have dates blocked beyond that rolling 6 month window. As the days advance and the calendar opens up future dates as it comes to them it does not recognise that certain dates have been blocked and simply opens them up for booking. This coupled with the ability of guests to use Instant book means that dates you thought were secure from booking in actual fact are not!
Get onto Airbnb and explain what has happened....they have heard it a million times before, and they will know what to do to relocate or take care of this booking. All the best Rachel....
Cheers.....Rob