The previous thread on this subject was shown as resolved and closed, however, my issue very much exists and is not the fault of any outside apps, such as Facebook messenger, which was determined to be the culprit in the previous thread.
I'm experiencing this same problem however it us occurring when using all Samsung messaging apps at both ends, sending and receiving. I'm receiving unreadable screen shots from my friend who is taking the screen shot with her Samsung S20 5G FE, sending it to me using the native Samsung messaging app, and I'm receiving the message on my Samsung S20 5G using the native Samsung messaging app. The screen shot she took is clear and readable on her end when she sends it to me, but what I'm receiving is fuzzy / blurry / pixelated to a level of being completely unreadable.
There's nowhere to run and hide on this one, Samsung. This is 100% your issue.