There are serious bugs in iOS 11 that break Construct games, so we default to iOS 12+ to avoid those bugs affecting your games. Unfortunately these issues were caused by Apple and they were only fixed with the release of iOS 12, so we have no good workaround to those problems other than to set the minimum supported iOS version to 12+. According to StatCounter this already covers over 90% of devices though, so it doesn't seem like there's much lost in setting that as the minimum supported version.
It's hard to comment on why the game relaunches without seeing a project that demonstrates the issue. It's possible it's doing something like exceeding the hardware limits of the device and crashing due to running out of memory. There might be more information in Xcode's log.