Subject of the issue

If I have an item in another grid or an item outside of the grid dragged into a new grid, it's attributes are not preserved.

Your environment

Version of gridstack.js: 2.0.0
Browser: Chrome on Ubuntu (Version 83.0.4103.61 (Official Build) (64-bit))

Steps to reproduce

1. Open https://gridstackjs.com/demo/two.html
2. Drag item with text "has maxWidth=3" from the left grid to the right grid
3. Open console and type "grids[1].save()" (saves/serializes the right grid item data)

Expected behaviour

The moved item should have its ID attribute saved in the serialized JSON.

Actual behaviour

The moved item does not have its ID attribute saved in the serialized JSON.