It has happened to me twice so far in the last couple of months and hosts on other forums report it has happened to them too. I am very careful about blocking out the dates. And then going back in on another server to check, so am 100% sure they were blocked and Airbnb acknowledged this was the case.
When I spoke to customer services, they acknowledged there was an known glitch and they would be able to check and see whether my calendar had been blocked and was then unblocked.
Airbnb emailed me half an hour later to confirm this was the case and they cancelled the booking.
It makes no sense for hosts to be penalised by Airbnb using their 'free cancellations' for a glitch that they acknowledge is their fault. I would perhaps use twitter to try and get some resolution?
Best of luck.