I’ve found that on mobile devices mouseDrag events are continuously called when touching the screen whereas on desktop devices the mouse is more “stable” so they only happen when actively moving the mouse.
Do you also get coninuous mouseDrag events when using a touch input on iOS? And do you get just one mouseDrag event along with the mouseDown when using a mouse or pencil, while you expected none?
Sounds familiar! That can be quite tricky. See the first point in this feature request: FR: Viewport improvements