Hi
@Pliggen It seems this info is a bit old but, boy do I got the solution for you!
The apps you want to keep on can be kept on if you long-press the app and go to (i) App info.
If your scroll a little you'll see the Battery option, press it.
You should have an option called "Background app management" <-- turning this off will keep your app from turning off while you have your phone on standby.
Please remember that this will eventually drain your battery more than usual since the app will indeed be running in the background.
Let me know how it goes!