Greetings from Syncfusion.
This is the current behavior of Gantt, in which start date, end date and duration values are interrelated as if one value changes other field values will be affected. We are doing some calculations while changing the values of these fields. If we prevent this automatic calculation, it will collapse the Gantt chart behavior.
But these calculations are only done for default start date, end date and duration columns. If we add custom column with date value, these calculations are not performed. For custom column, we need to define different field other than default field values mapped in taskFields property.
Please get back to us if you require further assistance on this.
Jesus Arockia Sankaran S