The horizon seems like a minor issue overall by logic but we are very sensitive to some features of our environment, two of those keenly detected subjects are eyes, any eyes are focused on in mass of complex scene elements, our brains key in on eyes. Creatures who could detect eyes ever when camouflaged, passed on their genes and those who couldn't faded from the genealogical record and were not our ancestors.
Another thing our brains key into is the horizon. Even a slight bit off changes our perception of the entire scene which is why landscape photographers use spirit bubbles to level the tripod mounted camera. Luckily most post-processing software allows for leveling afterward. Viewing a scene with a horizon, that is not level, causes discomfort, nervousness and affects the sense of balance. 1 degree off level is enough to cause discomfort.
From a perception view, the main tilting to the right takes the viewer's eye off the screen to the right instead of to his relationship to the people in the background left-center. When looking at a scene our eyes wander, alot, it is part of the scanning process that the brain uses to see the whole scene although the eye is at any given instant focused on a tiny portion of the scene. The constant scanning at different focal distances and light and dark sensitivity make for a composite of the scene to build up over a few milliseconds to give the impression of everything in focus near or far, and with detail appropriately illuminated by constant iris changes to give an impression of the scene in very wide range, like an HDR photo where very bright parts appear less so, and very dark parts seem less dark to make a 16 stop scene appear to be within the natural 10-12 stop range of our eyes. Modern cameras have more dynamic range than human eyes but they take a single frame at one aperture, ISO and duration. But the eye and brain work like a HRD shot plus composited panoramic photo, plus focus stacking image. That is why some claim cameras are not as wide a range DR and focus depth of field as the human eye. It is far better than the human eye if the camera and software merge many frames, each optimized for focus depth, aperture and panning, like the brain and eye do.
What does this have to do with art? Our capture of a scene with our eye works on these different principles and processes than a camera single frame and if artists want to capture the traits of a scene a human perceives in person, when painting from a reference photo, out to be taking lots of photos of different apertures, and depth of field plus stitching panned images so their reference is closer to what he would have seen in person.