I got the same problem as @Amandeep Singh in comment with the actual solution, but i found a workaround.

I'm converting the image to dataUrl immediately after loading :

fabric.Image.fromURL(url).then((img) => {
    img.src = img.toDataURL();
    canvas.add(img);
    canvas.renderAll(); // i need this to refresh the canvas in fabricjs 6 in my case
});

And by converting the image at loading, no need to extend toObject of fabric.Image