Reboot issues may be classified as the following,
Reboot in standby: The device freezes and doesn't respond when touched.
1. If the issue appears occasionally, it may be caused by anomalies in a file, it is recommended to observe the use.
2. If the issue appears often, check if the handset has access to root or not, if yes, please back up all the data, clear cache and flash to the latest OS version.
3. Make sure to Enter into "Setting"--"Advanced"--"Scheduled power on/off" is disabled

Reboot in third-party Apps
Please confirm if the issue occurs in one third-party App or other third-party Apps too.
1. If the issue occurs in one third-party App, please uninstall the third-party App, clear cache and check the latest update
2. If the issue occurs with other third-party Apps, make sure the app is compatible with the latest Android version. Try uninstalling the third-party Apps, please clear cache, back up all the data and flash back to the official latest OS version. Enter into"Setting"--"Advanced"--"Scheduled power on/off" to check if it is disabled.

Reboot in preload Apps
Please back up all the data, clear cache, uninstall the latest update(Enter into Setting--Apps--All to view), restore the factory setting or flash back to the official latest OS version.

Please confirm if the issue occurs in one third-party App or all the third-party App.