Apps are automated using Intruments. Once you leave the app, instruments crashes and doesn't let you automate anymore.
We automate Safari and Chrome using their separate debug ports (not Intruments) so even though Instruments dies, we can continue testing on the browser.
But you can't go back to your app once you leave, Instruments is done automating. This is how Apple's testing framework works currently.
It is possible that Apple could improve this and allow the operation in their new XCUITest framework. They have said they are very open to feedback, so go ahead and submit a report to Apple asking for the ability to test with XCUITest, and allow testers to leave the app and return to it.