Interesting. So that's the symptom of a memory exhausted error. The iOS kernel sends a SIGKILL message to your app and it shuts down immediately without being given even the ability to run any shutdown code.

The operating system already uses RAM up to a certain level. Given the device model (especially if it's old), iOS version, and the number of installed apps (especially apps that run background tasks or services), the available memory for new apps may be too low for yours.

At the moment I don't think the toolchain would be the culprit (until I am shown that the very same Xcode project, when built on macOS, doesn't exhibit the same symptom when deployed on the very same device). But the Unity engine is likely to consume a lot of RAM indeed. Maybe you're running your app on a hardware ecosystem that's out of its specifications ?