Unfortunately this is always a risk. iOS makes no promises about letting apps stay alive in the background, and will terminate apps throughout the day as the mood takes it. Though it’s become much more rare with newer iOS versions and newer phone models, it still can happen.
The best way to work around it is to install Arc Mini, and have that also alive in the background. Arc App and Arc Mini will share the job of recording into the shared database (so the timeline in both apps is the same, regardless of which app was recording at the time). If one app is terminated by iOS, the other app will notice within a minute or two and will take over the job. And while one is recording, the other will be in a standby mode that consumes effectively no battery.
It’s not an ideal solution. But short of Apple changing their policy on how they manage background apps, it’s the best we can do for now.