ClassName undefined for some child in iconCreateFunction

I'm working in a Vue2 web application using Leaflet and marker-cluster I have some issue with the iconCreateFunction option Here is a part of my tempate :

 <v-marker-cluster :options="{ iconCreateFunction: iconCreateClsPrg}">
          <l-marker ....> 
                <l-icon :class-name="programme.display ? 'custom-marker notDisplay' : 'custom-marker display'">

And here is a part of the iconCreateClsPrg method :

iconCreateClsPrg (marker_cluster) {
        const childs = marker_cluster.getAllChildMarkers();
         childs.forEach(child => {
            const cssCluster = child.options.icon.options.className;
            if (cssCluster && cssCluster.includes(this.css_marker_selected_simple)) {
                nbSelected++;
            }
          }
             ...
}

My problem is that sometimes my cssCluster const is undefined for no reason. It's not a regular behavior and it happens only at some zoom levels (which are not always the same). If someone has an idea, it would be a big help !

I tryied to refresh the map, to wait until undefined (but had some issu with async method), restart the method from scratch, use for instead of forEach, ... but nothing worked

I finally found a way to wait until the icon is fully loaded using promises :

iconCreateClsPrg (marker_cluster) {
            let nbSelected = 0;
            try {
                const childs = marker_cluster.getAllChildMarkers();
                const promises = [];
                childs.forEach(child => {
                    // create promise to wait for all child
                    const promise = new Promise(resolve => {
                        // wait for 0ms to let the browser render the page asynchonously
                        setTimeout(() => {
                            const cssCluster = child.options.icon.options.className;
                            if (cssCluster && cssCluster.includes(this.css_marker_selected_simple)) {
                                nbSelected++;
                            }
                            resolve();
                        }, 0);
                    });
                    promises.push(promise);
                });
                // wait for all child to be processed before continue
                Promise.all(promises).then(() => {
                    // do the rest of my things
                    const divIcon = new L.DivIcon({
                        html: htmlContent,
                        className: css_cluster,
                        iconSize: [30 + childCount, 30 + childCount],
                    });

                    // Ensure the map updates the cluster icon after the promise has resolved
                    marker_cluster.setIcon(divIcon);
                });
            } catch (e) {
                // deal with the error
            }
            // Return a blank icon to start with
            return new L.DivIcon({
            });
        }