Wow, I didn't know this. I just tried with iOS 14 beta and was surprised to see video/audio recording stop as soon as I hit the button to lock the screen.

By the way: You might want to try recording something while walking around with your phone in your pocket. My guess is that your clothing will muffle sound. Any movement will cause fabric to rub against the phone, drowning out the sound you actually want to record. Even if your tactic did work, I'm not sure it would have captured much of the conversation.