Which rounds the dragged task to the cell which is outside the visible part of a chart(so it looks like the task disappears). You can just disable this config, so the issue should be solved.
If it’s not your case, could you please provide more details on how to reproduce it, or reproduce it in our snippet tool:
https://snippet.dhtmlx.com/5/98dc2fab0
(open the snippet above => reproduce the issue on HTML/CODE/DATA tabs => click the “Share” button => send me the new link)
Get a guaranteed answer from DHTMLX technical support team
under the most suitable support plan