I have a newspaper app (Volkskrant, the Netherlands) which makes my iPhone 12 really warm when I use that app. I suspect a link to the “stuttering motion when closing an app” problem. since 2 days, I kill that App after using it. I have not experienced the stuttering anymore since then, while before, I experienced it multiple times per day. so my theory is that the stuttering motion happens because another app in the background (not necessarily the one you are closing) is consuming too much resources.