Error with Row-Level Security (RLS) in Power BI when using DirectQuery with Multiple Data Sources

I have a question about Power BI. I have imported a table (dataflow) into Power BI. On this table (Dim_Users), I am using Row-Level Security (RLS) through a filter function on a column that contains email addresses. I filter this column using the userprincipalname() function.

On my presentation page, I have only one table (Dim_Users) as a visualization, which displays all the users registered in my administration.

When I publish this and test the security as a role, it works as intended. The visualization only shows the row from Dim_Users that corresponds to the role I am testing.

However, when I add one table from another dataset via DirectQuery, without making any further changes, I immediately receive an error message when testing the RLS security on the only visualization (Dim_Users) shown in the report. The error code I receive is HTTP status code 401.

Any idea why I am encountering this error message and how I can resolve it?