In Web Viewer 12.1 exports open an Excel workbook starting with Sheet1. Web Viewer 14 export opens Excel with worksheet Sheet0 whether via a saved rule or not.
Configuration issue
When you use EXPORT_FORMAT = "OOXML"; (which is also default if unconfigured), the output will be an .xlsx file, and it will have Sheet 0.
If you use EXPORT_FORMAT = "XML"; in ConfigFile.cfg, the output of export will be an .xml file, the same format as used by Web Viewer 12, and it will have Sheet 1 as you expect.
See how to configure that in our online documentation:
Format for Exported Reports