Running App Stops Mid-Run

Hi there! I've recently been having a problem with the C25K running app on my phone cutting out, usually during the first run rep. This problem started after I downloaded Android 9 but also after a big jump in the length of the runs. I've tried reinstalling the app and I've tried an alternative app but neither has worked. Leaving the phone screen on is the only solution I've found but is not ideal as I don't want to put my phone in my pocket with the screen unlocked. If anyone has any ideas I'd be much obliged!