Please note that the tasks will acquire fixed duration, once edited on a chart that is configured with a task duration calculation Calculated based on start and end dates only. Meaning, task duration will be changed without affecting the task estimates in Jira. Thus, you may want to rely on this information when editing tasks via drag-and-drop or resizing.
Even though, task duration is switched to Calculated based on provided estimate, otherwise based on start and end dates, the described above behavior will be observed. In order to reset the behavior you would have to clear task duration on Edit dialog.