Roaming data leak on iPhone7

Travelling to US, I noticed that leakage of roaming data is taking place although "Mobile Data" option is switched off on my iPhone 7.

I'm being charged with high amount of roaming data usage by my mobile carrier back in my home country. Looking at the detailed usage report, I saw that both AT&T and T-Mobile (whichever I was roaming at the time) reported (to my mobile carrier) small amounts of (each one under 100KB) data usage in various hours of each and every day even though no calls/sms/data usage was taking place on those days.

I have a deal with my mobile carrier to use roaming voice/text/data for a certain fee in a per daily basis. That means even I use 1Kb or 1GB on a single day, I pay the same daily rate. This is why I'm being charged with a high amount although roaming data leakage is occurring in small amounts every single day. That's a total different issue about my plan that I'm going to follow up with with my mobile carrier.

Getting back to the leakage problem; while checking the detailed usage report mentioned in previous paragraph, I also noticed that the reported roaming data usage was following a pattern such that it looks like a negotiation or a keep-alive type of data exchange. It happens in long (like an hour long) periods and uses small amounts of data. Considering this, I conducted some tests by playing with iPhone7's mobile data options and I managed to locate the root cause for it by tracking actual usage reports of my mobile carrier.

As it turns out, setting "Voice Only" in "Enable 4G" option under "Mobile Data Options" menu is causing roaming data leaks even though "Mobile Data" option is switched off at the same time. Please also note that, roaming leakage happens even no any voice/sms/data exchange is happening even when "Mobile Data Usage" is shown as 0 bytes. In addition please also note that "Wi-Fi Assist" option is switched off on my iPhone7.

Long story short, I wouldn't expect iPhone7 to use roaming data when "Mobile Data" option is switched off (just to be on the safe side, instead of disabling "Roaming" only, I always disable "Mobile Data" when travelling overseas). It seems like a iPhone7 bug to me. Has anybody experienced same issue? Or anybody have any knowledge about whether this is a design intent?