Hi akhaqan,
Thanks for clarifying. We'd still recommend you adjust the settings with the maximization steps previously provided.
Some background activity is expected based on what the app is linked to. For example, location services or Bluetooth connections. As mentioned in the section "Use Background App Refresh" from the article here: Switch apps on your iPhone, iPad, or iPod touch
"After you switch to a different app, some apps run for a short period of time before they're set to a suspended state...With Background App Refresh, suspended apps can check for updates and new content."
If it's a large amount of background app data, you can also check and adjust your cellular settings for the app(s) with the help of the article here: Use cellular data on your iPhone or iPad and adjust any location service or Bluetooth settings as well.
Should you continue to have any issues, reach out to Apple Support directly.
Contact Apple Support
Cheers!