Thanks for contacting Syncfusion.
We have analyzed the reported issue by preparing the sample as shown in provided GIF file, but we can’t reproduce the issue in our sample. When the property updateTimescaleView in timelineSettings property is set as true, then timeline gets updated when edited task reaches the project start date or end date. When it is set as false, the timeline is not updated automatically on both left dragging and right dragging actions.
Please find the demo sample link.
Please modify our sample to reproduce the issue or please share us the issue reproduceable sample, so that it will be very helpful to provide you proper solution.