Qlik Sense: Safari requires authentication when switching back from another iOS app

This is something Qlik doesn't have control over: mobile devices, especially iOS ones, manage system resources on the fly.
In particular, iOS apps don't behave like PC or Mac software: when they are in the background, they stay active only as long as the operative system allows them to be.

Apple has defined iOS apps's cycle here: https://developer.apple.com/library/content/documentation/iPhone/Conceptual/iPhoneOSProgrammingGuide/TheAppLifeCycle/TheAppLifeCycle.html
If iOS needs to free up resources for another app, it will suspend Safari (or any other app). iPhones and iPads (even the so-called Pro version) are devices with very limited memory, so that will probably happen very often. Sometimes, this behavior can also happen when switching tabs inside Safari itself, due to the fact that the system still needs to free-up resources.

At that point, the Websocket connection won't be kept alive anymore: all selections will be lost and and, since iOS Safari doesn't automatically provide NTLM info, the user will have to authenticate again.