I have had similar issues with artifacts and have determined that this seems to be a problem where a large portion of my photo has a detailed structure. Here I believe the grass with water spots is creating the issue. Remove clarity and sharpness in the grass and resubmit your photo.
Artefacts is not detailed structure.
I see chromatic aberration and a contrast issue. There is also a noise issue. It's not to submit "completey unaltered pictures", but the pictures need to meet certain very stringent quality requirements. Take RAW pictures and edit them, export to JPEG, to meet those requirements.