[Solved] WPF ReportViewer: Initial rendering shows only 1 page, but clicking Preview renders all pages after Telerik Reporting upgrade

[Minita]

Hi Telerik Support,

We recently upgraded our application from an older Telerik Reporting version to Telerik Reporting 20.2.26.812 (2026 Q3), and we are experiencing an issue with the WPF ReportViewer.

Environment:

• .NET Framework 4.7.2
• WPF application
• Telerik Reporting: 20.2.26.812
• Telerik ReportViewer.Wpf: 20.2.26.812
• Telerik.UI.for.Wpf.AllControls.Xaml: 2026.3.812
• Previous Telerik Reporting version: 14.0.20.115

The report is a code-based report and has multiple pages. The first page contains patient information, followed by several additional pages/subreports.

The report is generated using an InstanceReportSource.

RefreshReport() is not called anywhere.

Problem

Before the Telerik upgrade, the report rendered correctly on the first load.

After the upgrade:

1. We click Generate Report.
2. The ReportViewer initially shows only 1 page, and the page is almost empty/incomplete.
3. The report should actually contain 4–6 pages, depending on the selected content.
4. If we click the Preview button in the ReportViewer's report parameter area, the report is processed again and all pages are rendered correctly.

We would like the report to render all pages correctly on the first load, without requiring the user to click Preview.

 

What would you recommend checking to determine why the second processing triggered by Preview produces the correct result while the initial processing does not?

The same report and application logic worked correctly before the Telerik upgrade, so we would prefer to identify the Telerik compatibility/configuration issue rather than modify the report logic unnecessarily.

Any guidance on what logs, assembly versions, or report settings you need from us would also be appreciated (As this is a healthcare application, I won't be able to share code or any sensitive information).

Thanks.