The most likely cause is lens flare or an internal reflection/ghosting artifact, especially if strong sunlight or another bright light source is near the frame. Similar curved arcs can appear when light hits the lens at certain angles, and they may change or disappear as you reposition the camera or light source. A lens hood can reduce this.

A filter is another possible cause, since filters can introduce similar arc-shaped reflections. Try removing any protective/UV/CPL filter and retesting.

One community answer also suggested a polarization-related artifact from the sky, since clear blue sky is partially polarized. That’s less certain, but it’s worth testing whether the effect changes with camera direction relative to the sun.

Good tests:

• Shoot the same scene while changing the sun’s position relative to the frame.
• Repeat with and without the lens hood.
• Remove any front filter.
• Try a different lens on the same camera.
• Try the same lens in similar light to see if the arcs are repeatable.

If the arcs track bright off-axis light, it’s most likely flare/reflection rather than a sensor defect.