Please see attached example (tested with version 5.1.2).
Try changing e.g. the name/title of a task and click "Reload data". Changes are reverted.
Try changing the position of a task (move it above/below a sibling) and click "Reload data". The task's position is NOT restored.
An update about that issue, is not a bug, for performance reasons, the React Gantt has the syncDataOnLoad config set to true by default, which will update the dataset and not replace it, if you set up that config to false, it'll work (I tested it here )