Subject of the issue

Elements vanish when dragging/dropping fast.

Your environment

gridstack.js H-5 v3.1.4
Mozilla Firefox 84.0.2 and Microsoft Edge 87.0.664.75

Steps to reproduce

Just drag/move/drop fast, you can use the official demo: https://gridstackjs.com/demo/two.html
Here a video: https://www.loom.com/share/008049b00f344f55ba8e3df90e17a50a

Expected behavior

The element will appear in the other grid

Actual behavior

The element vanish from the two grids