My account shows data usage even when I have data turned off on the phone. It is happening to both my pixel 3xl and iPhone. I ported out my pixel 3 line to Google fi and. It does show data usage when I have data is turned off on the phone. It is a very small amount, but I think something is not right on how your system is reporting usage.
My account shows cellular usage, even if it is turned off on phone. Only way to stop is to do it at device level