Instance "jumps" to mouse position

I've fixed the problem in a such way. It works perfectly on iMac, but then doesn't work on iPad.

What's the problem?

createjs.Touch.enable(stage);

this.L.y = 430;
let offset = 0;

this.L.addEventListener("mousedown", (evt) => {
let a = evt.currentTarget.parent.globalToLocal(evt.stageX, evt.stageY);
offset = a.y - evt.currentTarget.y;
console.log('mouse clicked');
});

this.L.addEventListener("pressmove", (evt) => {
let p = evt.currentTarget.parent.globalToLocal(evt.stageX, evt.stageY);
//evt.currentTarget.x = p.x;
evt.currentTarget.y = p.y - offset;
if (evt.currentTarget.y > 730) {
evt.currentTarget.y = 730;
} else if (evt.currentTarget.y < 430) {
evt.currentTarget.y = 430;
}
stage.update();
});

this.L.addEventListener("touchstart",function(evt) {
evt.preventDefafult();
let a = evt.currentTarget.parent.globalToLocal(evt.stageX, evt.stageY);
offset = a.y - evt.currentTarget.y;
console.log('touched');
});

this.L.addEventListener("touchmove",function(evt) {
evt.preventDefafult();
let p = evt.currentTarget.parent.globalToLocal(evt.stageX, evt.stageY);
//evt.currentTarget.x = p.x;
evt.currentTarget.y = p.y - offset;
if (evt.currentTarget.y > 730) {
evt.currentTarget.y = 730;
} else if (evt.currentTarget.y < 430) {
evt.currentTarget.y = 430;
}
stage.update();
});

Capture the X/Y difference between the mouse coordinates and the clicked symbol to a set of persistent variables, then subtract (or add) that difference when setting the symbol's dragged coordinates.