When exporting a report bundle each page in the bundle is exported to a separate worksheet. If you have individual reports that span multiple pages, this will result in that individual report spanning multiple worksheets when exported to Excel.
If you'd prefer the page breaks to occur only after each report, then you can enable the Optimise for exporting to Excel bundle setting. Essentially when using this setting for exporting a bundle, page breaks are used to create separate worksheets.
Browse to Reports -> Bundles and select the required bundle from the left sidebar navigation.
Click the 3 dots menu to the right of the bundle heading and select Edit bundle information.
Check the Optimise for exporting to Excel option and SAVE PROPERTIES.
Note: for details on other properties please refer to the Edit a Bundle help article.