When offline-exporting fails to export the chart on the client side, there is a fallback to the public Export Server instance, which can be either disabled or enabled. By default, it is enabled since certain older browsers might experience issues when exporting locally.

You can disable this fallback by setting exporting.fallbackToExportServer to false.

Reference: https://api.highcharts.com/highcharts/exporting.fallbackToExportServer