LightBox in Incognito Mode

I am trying to pass on data from a button on a page to a LightBox like so:

function button1_click(event) {
let datDateStart = new Date();
wixWindow.openLightbox("loadtest", {
"datStart": datDateStart

And on LightBox side like this

$w.onReady(function () {
let received = wixWindow.lightbox.getContext();
console.log("Start at:" + received.datStart);
let datDateEnd = new Date();
console.log("End at:" + datDateEnd);
var diff = Math.abs(datDateEnd - received.datStart);
console.log("diff=" + diff);

In every browser I tested, running in Incognito Mode, no data is passed on to the Lightbox.
Anyone ever ran into this?