It can also be something in your app’s code. For example, one of our users had a simple popup in the app, presented only at the first start of the app. Once the popup was dismissed, the fact was stored in the app’s local storage, and the popup was not shown anymore. They did dismiss the popup on their iOS Simulator, but on Bitrise every build runs in a brand new, clean environment, which means that the simulator is in the same state as if you’d hit “Reset Content and Settings” in the iOS Simulator’s menu.
Try to clean out the simulator/emulator before running tests on your Mac/PC, to simulate the “first run” experience.
Occasionally, a build just doesn’t generate any log output. This can happen for various reasons; you can find an example in case of an iOS library project.