Sending screenshots to other people using apps like Instagram and Facebook massively degrade the quality of the image once sent but the image is in high quality in my gallery.
Are you sharing the screenshots from the screenshot menu that appears after you take the screenshot? If so, try opening the samsung gallery app or google photos app and sharing it from there.
That's normal behavior, those programs compress images when they send them to reduce their size. Some of the photos I upload look terrible compared to the original, a lot of detail gets lost.