When using the latest version of the Kendo Default Theme:
<link rel="stylesheet" href="https://unpkg.com/@@progress/kendo-theme-default@latest/dist/all.css" />
the export in the Blazor Report viewer does not work.
Meanwhile, you can reference the older kendo CSS styles directly in the view with the ReportViewer. You may leave the new styles referenced on the layout page. It works with:
<link rel="stylesheet" href="https://unpkg.com/@@progress/kendo-theme-default@4.26.0/dist/all.css" />