Vaadin Flow Treegrid - empty rows after expand

Question

I use a TreeGrid in Vaadin 19.0.6 and add items like this:

final TreeData<BEAN> treeData = dataProvider.getTreeData();
treeData.addItems(items, getChildren::apply);

When I select a root-node which contains children and after that expand this root node, the children are expanded but the other root nodes disappear.

If I collapse/expand this node several times, the items are back again. Any ideas why this is happening?

Thanks for help! Daniel