I just noticed this issue on my iPhone 7, in 11.0.2. I imported some jpeg images from my Canon camera, and noticed that particularly red and green edges are all jagged and pixelated in the iOS Photos app.
Here’s a cropped screenshot from the Photos app, see the green bottle:
I checked on the camera itself, and it looked fine. Then I opened the same image in Snapseed, and you can see there are no “jaggies”:
Here’s a cropped screenshot from the Photos app, see the green bottle:
I checked on the camera itself, and it looked fine. Then I opened the same image in Snapseed, and you can see there are no “jaggies”: