See https://support.airtable.com/docs/how-to-add-and-configure-the-gantt-view#configuring-your-data
"Instead of using date fields, you can also use computed fields that output date values. However, if you use computed fields, you will not be able to manually adjust the start or end dates for your records by dragging the records on the Gantt chart."
Unfortunately, using formula fields in either date disables the start and end date drag functionality for both dates. Dependencies still work. Instead of using a formula field to calculate the end date, you might consider an automation that copies that value of the formula field into a date field whenever the formula updates.