We've been noticing that even though we have cellular data turned off for specific apps (like YouTube) that if the w-fi signal cuts out, cellular data is still used. Is this a new bug in iOS 10, or is there another setting somewhere that we are missing? We also have background app refresh turned off, and wi-fi assist turned off. We've noticed it on several apps, but yesterday YouTube managed to use 1GB of data with the cellular data setting off specifically for YouTube. (Our home wi-fi router had quit working and we didn't notice.)
Short of turning on airplane mode at home, what else can we do, or why is the "use cellular data" setting not working for some apps?