RichTextField - contentsCss in configJsFile not working

When using the following configuration for a Rich text field in magnolia 6.2 the "contentCss"
is not used in the head of the ck editor iframe.

However when using the same configuration in magnolia 5.7.5 it is working as expected.

CKEDITOR.editorConfig = function (config) {
    //Toolbar Layout
    var toolbar = [
        {name: "clipboard", items: ["Cut", "Copy", "Paste", "PasteText", "PasteFromWord"]},
    ];
    config.toolbar = toolbar;
    
//CSS classes
    config.contentsCss = ['https://frontend.live/unionInvestment-website-frontend/develop-v4/resources/css/styles.all.min.css', 'https://frontend.live/unionInvestment-website-frontend/develop-v4/resources/css/styles.all2.min.css'];
};

there are no frontend, nor any backend errors logged.

In Magnolia 6.2.1 I see the following ckeditor in the dialog. This dialog shows that the toolbar configuration is working:

When looking at the html code, the configured resources (contentCss) are missing in the head of the iframe:

Also when using the console to display the CKEditor variables the two entries "
https://frontend.live/unionInvestment-website-frontend/develop-v4/resources/css/styles.all.min.css
https://frontend.live/unionInvestment-website-frontend/develop-v4/resources/css/styles.all2.min.css
are missing:

Now when we look at magnolia 5.7.5 we see that it was working in this magnolia version:

You can see that both css files are included in the head within the iframe:

and here the console output of the variable: