I have the exact same issues as everybody else and even now the issue still plagues all iPhones and onto the new iPhone 12...
It is a problem of the apps refreshing in the background for some reason iPhone processors are going to sleep before the background refresh settings gets to refresh etc, mail, camera etc etc, or whatever the app you are using at the time.
Personally, I haven’t found a workaround as it is a firmware issue so in apple’s court for repair and it isn’t new but they are sitting on there hands as they are more involved with the phone than firmware repair sad but true.
So, playing with settings like turning off background refresh in the settings and restart iPhone, gave me a few days of no frozen apps in background, that’s about it.