This app doesn't require INTERNET permission, so your usage data is completely secure. Refer to https://dontkillmyapp.com/ if this app is not tracking usage as per your expectation.

- Enable app usage settings for this app when the pop-up shows up initially. Else go to Settings menu -> Enable Settings -> Provide app usage permission.

- Long press on the app in the usage report to add them to the Keep On Track list. Those apps will be tracked even if device is locked.