The sensors in phones do drift a lot. They basically realign pretty much constantly to the local gravity vector, which is kinda bad in long turns.
of course, the kalman filters do not have the latency built in to adjust for turns like the ac does, or in a climbing/descending turn...
Update rate on the iPhones/iPads is a real issue, as they are constantly searching and trying to connect, thus burning battery rate.
We are running the iPhone 8, the same stuff, even when connected to the ac wifi/bluetooth.