Help please

New camera and lens so I'm relearning a few things.

5Ds, EF 16-35mm F4 L with a hood and lens protector. In this pic there's an arc (what's the technical term?) in the lower RH corner. The rising sun is to the far left. I assume the arc has occurred because of some refraction (?) caused by the lens protector.

My previous setup was a 7D with an EFS 10-22. I never had this issue but then I didn't use the protector as I often would fit a Lee filter holder.

Please enlighten me.........