Good news is, it's not just for Samsung phones. Everybody whose using whatsapp regardless phone models are having the same issue. Maybe you don't notice it earlier due to the picture itself, which maybe has less highlight and details.
I also experience the same problem with my A50. When i post a photo on my status for the 1st time, it usually retains the quality.
The problem comes when its the second upload time while the photo has been in the phone for a while. In the gallery, it remains of the right quality.
I really hate this😒😏