Trip logging WILL STOP if the app is force-quit manually by using any of the methods found here: https://support.apple.com/en-us/HT201330
The MileCatcher app needs to stay launched in the background in order to log your trips.
If you want to confirm that MileCatcher is launched and running in the background, you can check that the MileCatcher app is visible from the App Switcher. If the MileCatcher app is missing from the App Switcher, relaunch the app and keep it launched to resume trip logging. Instructions for how to view the App Switcher can be found here: https://support.apple.com/guide/iphone/switch-between-open-apps-iph1a1f981ad/ios