I had the same issue even with one UI 7 may update the only fix I have found is by going on developer options near the bottom where it says app , go to background process limit and turn standard limit to no background processes is the only way I have found to get round this just now I know the issues will get fixed in further updates but this has helped my battery and stopped it heating I'm not getting around 9 hours of on screen time again I did get more before may update but hopefully it will get better when all the issues are fixed