Unfortunately, this is by design...Airbnb are quietly doing an experiment with our listings.
Here is what they finally emailed me a few days (and a few back and forth communications) after I noticed the issue:
"We make ongoing changes to the search page and features available to help travelers find the listings that are most relevant to their needs and desires. That often includes experimenting with the placement or existence of different buttons or filters, and gathering associated data to see what our users like best. At this time, we're experimenting with search results for valued hosts and listings which is why your listing is showing up for less than 3 days in upcoming months, this will allow you to take on bigger reservations on following months, however, your listing will also show up for shorter reservations if they are made in the current month (ex: the listing is shown for one night in May since it is the current month).
We are always working to improve our products and policies, and we encourage you to submit your feedback in direct reply to this email. Even when unable to accommodate all requests, Airbnb truly values user feedback -- your voice is both powerful and essential. I’ll be sure to pass your thoughts on to the right team.
Thank you for your time and contribution."