Even when you are taking the photos at a 16:9 aspect ratio, the sensor will record the photo at 4:3 and and the camera.app will just crop the photo. The sensor is 4:3, so the photo will be taken as 4:3 and then cropped.
When I take a 16:9 photo, iCloud Photos will sync it to my Mac as 16:9, but it will show in Photos for Mac as an edited photo, and when I revert it to the original it will be 4:3. I am having the same iPhone model as you and as far as I can tell, t has always been this way, that the camera has just applied a cropping to the 4:3 original. It has been saving us an editing step.