The latest versions of Android 9.0 Pie disable location services when the app is not active or the phone is locked while battery saver is enabled. This prevents FAIRTIQ to determine your journey and results in the app automatically ending your journey.
In order to not be checked-out by the app and to keep a valid ticket, you need to disable power saving mode in the system settings while using FAIRTIQ:
Go to Settings → Battery and disable the Battery Saver feature.
Additional note about certain Huawei and Xiaomi phones:
Some Huawei and Xiaomi phones have their own energy saving mode that disables the location services required by FAIRTIQ to determine your journeys. To solve the problem on these devices, you have the option to make FAIRTIQ a protected app. To do so, open Settings → Power Saving → Protected app → turn the service on for FAIRTIQ.