It happens when Airbnb updates the websites and you are accessing the site and updating your calendar through something other than Google Chrome. Google Chrome is a more universal web language and what they recommend we use-- as a complete rebel, I am a Mac and Safari user since the earth cooled, and refuse to change to Google Chrome-- it has happened to me a few times.
I have started taking screen shots of my blocked days, with notes that indicate personal time off-- so when this happens, and it always does, I can contact Airbnb and have some documentation as back up to have them cancle and reroute the guests-- It happened last week.
First, if this happens, don’t freak out, and do not cancel a booking yourself-- Always get help from Airbnb when this happens, lest you have to deal with the more difficult issue of having your entire calendar shut down-- Because this is a known issue, they are always more than happy to help you out.