I have November set with a 2 night minimum. December is set with a 30 night minimum.
A guest is able to book as many days as they want in December (30 night min) as long as their reservation starts in November (2 night min).
I just had this happen and so far airbnb support isn't being helpful at all. A guest booked the last day of November with a 2 night minimum into the first week of December with a 30 night minimum. I tested it myself with Rule Sets and also Custom Trip Lengths. The first day of your reservation is all that matters in terms of minimum night stays.
Has anyone else had this happen?