I have the same issue on my S21 Ultra from time to time @S786
when recording video. When I record video on the 10x telephoto lens the image can be shaking noticeably in the viewfinder. Strange thing is though that when I play back the video it is smooth and perfectly stabilized. I keep my electronic image stabilization mode on though so maybe this helps here?. I find that after some software updates this issue goes away completely only to return after another update. There will be the same slight camera shake when taking pictures using a high zoom before the very high magnification stabilization lock kicks in as this is to be expected. Are your pictures and videos you take whilst there is shaking in the viewfinder coming out okay S786?..