EDITED TO ADD: I figured out the problem, and it’s not a bug in the game. Because the game is in landscape mode, one has to swipe from the long edge, in landscape orientation, to return to the home screen. Maybe I should leave this here in case someone else had the same confusion I did.
I just bought this game for an iPhone XR, and once I launch the game I can’t leave it.
The iPhone XR doesn’t have a home button, but uses a swipe gesture to exit an application to the home screen. This gesture doesn’t have any effect when MiniMetro is running. I can reach some iPhone pages like the notifications page, but nothing that allows me to reach the home screen or force-quit an application.
Right now the only way out is to shut down the phone and reboot it.