Ensure text remains visible during webfont load --> Font-swap not working

Hi,

I have the following isssue in Google Lightspeed : Ensure text remains visible during webfont load

As mentionned on other topics, I've added the following function that should solve the issue :

add_filter( 'generate_google_font_display', function() {
    return 'swap';
} );

But I am still getting the error.

Can you please help ?