This is just another one of Airbnb's plethora of known long-term "glitches". It's been around for years, surfaces randomly and intermittently. I've posted about it several times, reported it countless times - never gets fixed.
If you notice this on your account, you need to report it to CX, every time, regardless of whether or not you've received a booking at the lower rate. This is to cover your own arse because if this glitch appears on your calendar once, it's highly likely to pop up again and again in the future. When (not if) you eventually receive a booking at a stupidly low rate, Airbnb CX will invariably try their level best to convince you that it's your error, and that you must have neglected to update your calendar, and more often than not, they'll try to force you to take the booking at the silly price, even if you're making a loss on the booking.
The only defence you have is to tell the CX agent to check back on the tickets you've opened with them previously, which will prove that you already reported this exact "glitch" on your account to them (and always keep your own records of all reports/complaints you make to CX, for future reference) This leaves them with no choice but to accept responsibility on Airbnb's side, and cancel the booking with no penalty to you. Well... no penalty to you, unless of course they decide to use their latest little wheeze to "encourage" hosts never to cancel a booking, under any circumstances, by sneakily turning off your Instant Book ability for an indeterminate period of time...
