Oh I know what you mean, I have had so many problems with phones lately and the s21 is the best phone I have ever had and its perfect, to be honest it's happened before on other phones, one iPhone stopped charging in the night once when we had that 40c summer in 2019, my s20 did it on android auto, I think it's just the fact that it came up with a warning and the rails and camera housing felt very hot (as the s21 has a plastic back) and the battery just seems to be going down quicker but I think it's just my mind has I have been on it alot since and I find the second half of the battery always goes down quicker than the first