To reproduce:
Bind the tree to the following structure:
public class DeskSites
{
    public string DeskSiteName { get; set; }

    public Channels channels { get; set; }
}

public class Channels : List<Channel>
{
    
}

public class Channel
{
    public string ChannelName { get; set; }

    public FileSets fileSets { get; set; }
}

public class FileSets : List<FileSet>
{
   
}

public class FileSet
{
    public string FileSetName { get; set; }
}
Workaround:
Use the the generic class instead of the inherited one:
public class DeskSites
{
    public string DeskSiteName { get; set; }

    public List<Channel> channels { get; set; }
}