I can tell you a few things of my experience over the last year or so with iphones and Verizon data usage.

I have observed on multiple occasions where Verizon billed for more data than was indicated by the iphone usage. Verizon gave me some free data for a month or two but never explained why it occurred. They never responded to the technical ticket that was opened on the subject. In recent months, it appears that the problem is no longer occuring.

Additionally I have seen data usage on multiple phones at times when the phones are on wi-fi and the user is asleep. Bear in mind all the settings such as wi-fi assist, no background app refresh etc. are in the correct postion. The only fix that has worked to date is turning off cellular when we go to bed.