Why is a corner of InSight's selfie cropped like this?

A couple days ago, InSight took a selfie from the surface of Mars:

^{Image credit: NASA/JPL-Caltech}

Most articles about it note that the photo is a composite image, which you can figure out by looking at slight changes in the hues of the Martian soil in the top half of the image. I’m aware that Curiosity and MAVEN have also used this technique.

Many parts of the edges of the picture are straight lines, showing the edge of each image. The shape is presumably irregular because of the different angles from which each image was taken. However, the top right corner surprises me:

It looks much more curved that the other visible sides, almost as if either lots of smaller images were taken (and I’m sure that’s not the case) or something was cropped out – or perhaps there was an issue with image quality for some reason.

What’s the reason behind the irregular shape at this edge of the selfie?