The user is correct with the line around the image. It's a halo caused by the phone over processing the image. The S21U using the Exynos processor doesn't seem to handle image processing very well at all compared to the Snapdragon processor. I've seen side by side comparison online and the Snapdragon is far ahead in most things on the S21U.
Samsung need to a button to turn off the post processing of the image when using 10x, 30x and 100x zooms. Turning on/ off the scene optimisation (ai) doesn't work and no matter what settings you play with, the phone will always over process (over sharpen) the image a few seconds after taking it.
To show this, take the image and quickly switch to the image and you'll see the original image suddenly change to over sharpened mess literally before your eyes. You have to be quick to see it.
In my opinion the user should have full control over processing and not the phone. In some instances the image looks fine until a few seconds pass the image changes to over sharpened and a halo (white edging around the subjects).
I didn't have this issue on my P20 pro which took consistent decent photos.
I've also noticed during sunsets or sunrises the S21U over saturates and suffers from severe lens flare, again something my P20 pro just got on with and did a decent photo with no nonsense. Didn't have any lens flares or green dots like the S21U does everytime . This is a well documented issue of the S21U if you search online with thousands of users complaining about it.
