Android Battery Optimizations
Since Android 6.0 Marshmallow, apps running in background are controlled and restricted by the OS. By default, apps are not allowed to run in background, thus 3C Toolbox or 3C Battery Monitor will cease monitoring when their UI is not visible!
When you configure a service that runs in background (battery monitoring, notifications, widgets, line indicators, recordings, etc), the app should ask you to exclude itself from battery optimizations. In case of doubts, open app UI, open left navigation panel, settings (at the top) and select battery optimization. From there, find the app and exclude it (not optimized).
Alternatively you can add a notification shortcut that will tell the OS the app is "visible" and running in background. That should help keep any monitoring or services active, at least on stock Android.
Please note that many manufacturers are modifying this mechanism with their own, and apps cannot ask for exclusion from such, it must be done manually.
If the above and below for non-stock Android, are not enough to keep app running, please send a support request from app settings, help and support to get help with this issue.
The following screens show how to manually disable those restrictions on MIUI 10. There are actually 3 options that can be enabled, battery saver, autostart and in other permissions, possibly to be started by other apps.
The following screens show the same on Samsung devices. (coming soon)