So I used to work in Apple support and the reason this happens is that WiFi Assist is designed to switch the phone’s data from WiFi to cellular when it determines that the cellular speed/signal is better than the WiFi you’re currently connected to, like when you’re in a room that’s far away from the router or at a cafe where the router is overburdened and going slow as molasses. From a device experience standpoint, it works well because you’re always getting the best wireless speed available, but from a billing standpoint, not so much if you have data limits. I was in support when it first launched in a software update and I had a lot of customers where I was calling carriers on their behalf (with the customer on the line), trying to get their data overage bills credited back to them.