Assistive touch button disappears from the screen every now and then. When you go to Settings -> Accessibility it shows it's On but the button doesn't show up on the screen. You have to disable and enable again for t show
I think this is a bug with iOS 12, my phone does the same thing but only after sitting overnight...i have to use Siri to get into the accessibility section, turn assistive touch off and then on again for it to reappear.