Hello,
We are aware of this bug. That issue occurs because of the keyboard_navigation extension. After you type something, Gantt is repainted, and the keyboard navigation tries to restore the focus to the grid header cell, not to the element inside it. The dev team will fix that issue in the future, but I cannot give you any ETA.
As a workaround, you can switch the focus to that element after the render method: https://docs.dhtmlx.com/gantt/api__gantt_render.html ;
Please check the following example: https://snippet.dhtmlx.com/im7d45i5
Also, you can use the onchange event instead of the oninput event: https://snippet.dhtmlx.com/ns21nd02
Another way is to turn off the keyboard_navigation before repainting data, then turn it on: https://snippet.dhtmlx.com/42bcnqeq