Hello,
When you export the data with the raw: true parameter, Gantt saves all HTML elements of the Gantt container and sends them to the export server: https://docs.dhtmlx.com/gantt/desktop__export.html#parametersoftheexportmethods
So you need to modify the column configuration before exporting the data, then revert the changes.
Another way is to backup the column settings, modify the existing settings, export the data, and revert the changes: https://snippet.dhtmlx.com/9nuwfdtp
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