Get DisplayField from Layer loaded via WebMap

[SebastianKrings]

Hi,

we are loading our map from a WebMap which also contains all layers defined as part of the webmap via portal.

const webMap = new WebMap({
    portalItem: {
        id: defaultWebMapId
    }
});

await webMap.load();
map = webMap;

Unfortunately the field "DisplayField" is undefined for every layer.
This is how a layer looks on the service:

Why is it empty in JS after loading the webMap?

How do I get it otherwise?