I have my listing set so it won't receive reservations more than 90 days out. Every day, it unblocks a day at that borderland. It seems like when they release the day internally because it's no longer more than 90 days out, that unblock overrides the dates I have manually blocked in the calendar.
Because I have limited availability, this pattern has become obvious to me. If you only blocked dates occassionally, it would be a huge mystery. I've brought the issue to the attention of AirBNB and they told me to clear my browser data. I've explained in more detail and am providing screenshots, but they haven't even been willing to tell me the status of my ticket.