I don't know, but I believe I know how you can find out. Login to your Chase account, look at your Ink Preferred card, and see how Airbnb coded. If it's Travel, you should get 3x. I suspect it is not Travel, and that's why you didn't.

My opinion is that Chase should ensure that customers have a method to learn how a merchant codes before making a purchase. There was a tool at Visa that allowed this, but it died a few years ago.