I agree with testing in Safe Mode. All Safe Mode does is to temporarily disable any 3rd party app you installed. This can help to determine if the problem is due to something you installed vs something with the system. Even though the phone says "System Apps" are using all the data, it could still be some 3rd party app that is utilizing a system app inappropriately. You won't lose any personal data. To get out of Safe Mode, simply reboot the phone normally.