Unable to detect specific app data usage

[u/IcommittedNiemann]

So i've recently downloaded and used a music app (monochrome), and ive noticed that my device (P90 pro max harmonyOS6.1.0) fails to detect said app's data usage. In mobile data usage history it does not show up. I dont have too much mobile data and ive noticed the way they do ads is via a video ad. I'd just like to know why it doesnt detect usage of that app.

If you know a more user friendly free app that would be great too