To reproduce: please refer to the attached sample project and follow the steps in the gif file. 

Workaround: 

        private void radRichTextEditor1_CommandExecuting(object sender, Telerik.WinForms.Documents.RichTextBoxCommands.CommandExecutingEventArgs e)
        {
            if (e.Command  is ChangeIntegratedWatermarkTypeCommand && this.radRichTextEditor1.LayoutMode!= Telerik.WinForms.Documents.Model.DocumentLayoutMode.Paged)
            {
                e.Cancel = true; 
            }
        }