The iphone 7 on our account turns hotspot off when out of range of wifi but still showing users connected. In fact the users are connected but hot spot icon is off. Huge amounts of Data used. My Wife ran out of Data of 15 GB after a few days, her normal usage is 5 to 10 GB for the Month. Turning airplane mode on will disconnect all users.
This appears to have started after an update last week (Ver 12.0.1) at least we never saw the problem before and I know we never used this much data before.