It might be because it's a "new iPhone 16!" It takes a while for Photos to scan through all your pictures, note things that might help with Memories and Trips, and then generate the Trips views. If it's been less that a couple of weeks that you may need to wait a bit more.
Make sure that "Show Featured Content" is turned on--that's available in the Apple ID button at the top right.

If you're new to iOS 18, be sure to look around in Settings>Apps>Photos, as well as the above button, for things you might want turned on or off. If you haven't upgraded to 18.4, you should do that.