de-Bayered L-MastCam, the dark seam shadow (lower middle) is an artifact of the ‘subframe’ images that was not removed by the stitching process due to a small overlap.

Curiosity’s L-MastCam can no longer acquire full frame images after its filter wheel jammed half way open. Here’s a link to the frame the sensor now sees. So they need to take more images that are cropped into rectangles and then assembled into what you see above after they are Bayer reconstructed. https://mars.nasa.gov/msl-raw-images/msss/04289/mcam/4289ML1063190061705053C00_DXXX.jpg

Assembled in MS-ICE

Assembled from 16 overlapping images

NASA/JPL-Caltech/MSSS/fredk