I shot the photo below of some brown pelicans flying over on a Nikon P950. They were against a bright sky, so the silhouette is not surprising. I imported the raw into Lightroom. I lowered the exposure -1.67, which looked right, then did highlights -100, shadows +100, whites up until something saturates which was +86, blacks down until something saturates which was +2, which is my usual process. There is a dark halo around each bird, which remains with any change to the highlight/shadow/whites/blacks sliders, though it is less noticeable around shadows -60 but back at shadows-100. It is less noticeable on the post to stackexchange than on my computer. What is going on here? The original is in Dropbox.
Operations that increase contrast often produces halos at high contrast edges.
Sharpening will also produce halos at high contrast edges.
When the halos detract from your intent, it can be a sign of over-processing the image. Though it is worth keeping in mind that intent can vary. Low resolution images tend to tolerate higher degrees of sharpening and contrast than high resolution images.
(Also, there appears to be some chromatic aberration on the edges as well when I zoom in. This is a result of both the character of the lens and not applying a mitigating filter in processing).