Headspace stopped playing after I locked my iPhone screen

I’ve been using headspace for a long time. Starting yesterday, when I played a sleep cast and lock my screen, it will stop playing after a few minutes (it does not stop immediately when I lock the screen). It wasn’t like this before. I tried restarting my phone and reinstalled the app. Nothing helps. This doesn’t happen to other music apps. If the screen is on and I switch to another app, headspace will continue to play for a couple minutes and then stops suddenly.