The lines are most likely edge artifacts from in-camera processing, especially excessive sharpening. Sharpening boosts local contrast at boundaries, making the dark side darker and the light side lighter; when pushed too far, it creates visible dark or bright outlines (often called halos).

A second possible contributor is hard, direct flash close to the lens, which can create very thin dark shadows at object edges. But based on the described gray/black edge outlines, over-sharpening is the stronger explanation.

To reduce it:

• Lower the camera’s Sharpness setting or use a less aggressive picture style.
• Shoot RAW so you can control sharpening later.
• Apply sharpening more gently in post-processing.
• If flash was used, try diffusing or bouncing it, or add fill light/reflector to soften edge shadows.

In short: these are usually processing halos from too much sharpening, sometimes made worse by harsh direct lighting.