We've been hosts for almost 3 years with 60+ rentals and just had this issue come up...
We had an inquiry for 3 nights and pre-approved the guest for 3 nights. The guest was then able to Reserve our house for only 2 nights - we received no notification of this change, doesn't show up in the messages, no re-approval required.
From AirBNB's help page: "Once pre-approved, the guest can automatically confirm a booking for the specific dates provided."
This normally wouldn’t be a problem, but the number of nights is often, as was this case, a factor in negotiating a late checkout.
Is this a new bug? Someone could negotiate a great per night deal with for a high number of nights, then change the number of nights. Right?