Hello, I did not manage to reproduce your issue. Here are the steps I took.
- Logged into https://dev.powerbi.com/apps and registered a new application of type Server-side Web app with Redirect URL http://localhost:13526/Redirect
- In Visual Studio, compiled the project https://github.com/Microsoft/PowerBI-CSharp/tree/master/samples/webforms/embed-a-tile-into-an-app (entering my Client ID and Client Secret from step above).
- In Power BI Desktop, generated and uploaded a new report (since you report that the issue affects newer reports only).
- In Visual Studio, ran the project into Internet Explorer.
- In IE, followed the steps on the page. I was able to embed the report successfully in the IFRAME. The link looks like https://msit.powerbi.com/reportEmbed?reportId=fbba0813-cf35-4004-97c6-825c45869024