Prevent iOS 11 from closing background apps?
This is a known bug in iOS 11 and should be fixed with iOS 11.1
Most of the Apps are using the BackgroundFetch functionality, which is totally broken in iOS 11

Currently there is iOS 11.1 beta 4 released. It seems that Apple will release the final version within the delivery of the iPhone X. 

If you want to have more technical details you can have a look at the technical topics in StackOverflow: iOS11 swift silent push (background fetch, didReceiveRemoteNotification) is not working anymore



UPDATE 2017-10-31

Just FYI: Apple released today iOS11.1 -> the issue should be now resolved from a operating system point of view.