A few non-rooted (no rooted) Android 7 and 8 phones from (Huawei, ZTE, Neffos and Samsung) were tested in this simple firewall (NetGuard) experiment. A selection of apps (including apps from the phone manufacturer) was chosen to be blocked from accessing the Internet regardless using Mobile Data or WiFi, by using NetGuard.
Before the experiment, NetGuard and GlassWire for Android were installed. The GlassWire is activated to monitor if these apps still can access to the Internet. Throughout the experiment, GlassWire reported that some apps, that were blocked by NetGuard, had network traffic to the Internet.
Therefore, it seems that users who buy any Android phones are denied full control of the phone, even if they have installed privacy protection apps such as NetGuard. So, to make the experiment fair enough, this question is posted here, requesting help from experts out there, on whether what is the actual problem(s).
Is the problem caused because GlassWire reports false positives? Or NetGuard requires special configurations in order to block Internet for selected apps? Or Android phones disregard users and allow apps to access Internet even though NetGuard was installed?