Oh no - I forgot about this issue yesterday, took some new photos, added captions, re-arranged the order - all looked well when looking at the ad as a host. Then checked as a guest (preview), and all of the new portrait photos are cropped to landscape. My nice, new photo of the bathroom now only shows a piece of the toilet... My old portrait photos look fine, as do the ones I uploaded a few weeks ago.
Does anyone know why it sometimes foes not work, and how to fix it?
I can see from several posts here that this has bern an issue for YEARS, that Airbnb have been working on fixing - one would think that they would be able to solve it soon...