The RF 50mm f/1.2L USM vignettes at f/1.2. This is not a defect to correct in post. Left uncorrected, it is a compositional tool that draws the eye toward the center of the frame and applies a graduated darkening to the corners. Canon’s in-camera lens correction and Lightroom’s lens profile will eliminate this automatically if you let them. You should consider not letting them. Optical vignetting at the maximum aperture of a large-diameter lens is a physical consequence of the geometry: light arriving at the sensor from the edges at oblique angles is partially blocked by the lens barrel.