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?..
I think that smoothness of zoom is an area where a mobile phone will always fall way behind a good camera or camcorder S786. The shaking you describe sounds all pretty normal to me. I tend to use the pause record feature when physically in the act of zooming and letting record continue only when the viewfinder image has settled to keep my videos looking neat and smooth. It always amazes me that the IPhone to this day does not have a pause record video feature inbuilt. I could not get by without my pause record button. 👍👍