OS is caching some data in ram, it helps in faster opening apps, and since they are freezed, does not have impact on performance or battery life, so don't worry about it. In case of a short battery life you have to know 2 things: during first week or two of a new phone battery life is usually shorter, because phone need to learn how you use it. After some time it knows which apps you are using less and can faster freeze them or refresh them after longer period.
Second specific thing for Zenfone 8 is that it shows low battery state much earlier that it actually happens. You can check this by using phone till it turns off. After phone shows 1% of battery it will work for long time. Mine worked for 40 mins of watching YouTube since battery level was 1% till turn off.
