The TreeList renders its initial items expanded by default. However, if the Data collection is replaced, the new items will collapse.

The issue appears only when clearing the collection (or creating a new empty collection) and then adding the new items in it. If you create a new collection reference based on the current collection of items, there is no problem and the new items appear expended.

Reproduction: https://blazorrepl.telerik.com/wJEbcXvb207dv5vU11.

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TELERIK EDIT: You can override the Equals() method of the TreeList model class to prevent the issue:

    public class Employee
    {
        public int Id { get; set; }
        public int? ParentId { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public string Position { get; set; }

        public override bool Equals(object? obj)
        {
            return obj is Employee && ((Employee)obj).Id == Id;
        }
    }