In the Battery Settings, tap “Show Activity” you will see how much minutes the app usage in background. Normally, if you have Background App Refresh disabled, you will see some background activity after using some apps. This is usual for some apps with background activity up to 5 minutes after use. However, if the background activity is more than on screen time then the app went rogue and the system couldn’t completely suspend the app so you will need to force close it.