I'm experiencing the same issue.
Up until this year, I've been able to have people check in and out on the same day. They check out at noon and new renters can check in from 16h00.
Suddenly, for next year, that does no longer seem to be an option.
If I block a day as 'booked', it can not be booked for a following period any more.
I contacted Airbnb about it, but they claim this approach has never been a possibility. Nevertheless, I have been doing it this way for the past two years or so.
What's even stranger, is that it IS possible to make a proceeding booking up to the day where I already have a booking for the following period.
So, it is possible to book from February 27th to March 3rd, and from March 3rd to March 6th (existing booking), but not from March 6th to...
Does this makes sense to anyone?