Understanding why rls breaks

I have little experience with RLS.

I have tables: Fact, Dim1 and Dim2.

Everything works fine: I have RLS on Dim1 to filter based on USERNAME().

The RLS "breaks" when I introduce this measure…

Measure:=

IF (

    ISINSCOPE ( 'Client'[Name] ),

    MAX ( 'Client'[Type] ),

    CALCULATE (

        FIRSTNONBLANK ( 'Client'[Type], 1 ),

        FILTER (

            'Client',

            'Client'[Legacy_ClientNo] = 'Client'[ParentNo]

        )

    )

)

(Client would be Dim2)

It still filters Dim1 and the Fact correctly! But then I get all values with Dim2 (the filter works because all the values from the Fact for those Dim2 rows appear as blank!)

Why it breaks? Do I have to  apply the RLS on Dim2 too?