It's a known, long-term glitch @Thomas247. It's been around for years. Anyone who's experiencing this glitch, absolutely must call Airbnb and have them log that it's appearing on your account - even if there's no harm done, so far - because if you do receive an IB at any point, for dates you had manually blocked off, it's the only way you can prove to CX that it's Airbnb's error, not yours (they'll invariably try to convince the host that they must have unblocked the date themselves, and try to force them to honour the booking, or face penalties)
Also, it's always a good idea for hosts to screenshot dates they have manually blocked, as extra proof, should they become unblocked by the system. (Yes, I know it's ludicrous that one should have to go to such lengths, considering it's the website of a $38 billion tech company we're dealing with, but that's how unstable and insecure the platform is, unfortunately)
Airbnb UK - +44 203 318 1111