Workaround: handle the CommandExecuted event and manually adjust the location of these forms
public Form1()
{
    InitializeComponent();

    this.radRichTextEditor1.CommandExecuted += radRichTextEditor1_CommandExecuted;
}

private void radRichTextEditor1_CommandExecuted(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutedEventArgs e)
{
    if (e.Command is ShowInsertSymbolWindowCommand)
    {
        ((InsertSymbolDialog)this.radRichTextEditor1.RichTextBoxElement.InsertSymbolWindow).Location = Screen.FromControl(this).WorkingArea.Location;
    }

    else if (e.Command is ShowFindReplaceDialogCommand)
    {
        ((FindReplaceDialog)this.radRichTextEditor1.RichTextBoxElement.FindReplaceDialog).Location = Screen.FromControl(this).WorkingArea.Location;
    }
}