Before I explain how to fix this, let’s take a look at what’s happening behind the scenes.
When firstname.lastname@example.org opens the dashboard a connection string is created including the effectiveusername property, which is expected behavior.
The value specified for this property is email@example.com.
The connections string including the queries are sent via the On-Premises gateway to the SSAS server that hosts the data needed to view the report.
Once the connection is established, using the username and password specified in the Data Source settings, all queries are executed firstname.lastname@example.org.
Read on for the solution.