'Decrement paragraph left indent' command can set negative left indent which makes bullets/numbering of a list clipped. Instead, it shouldn't be executed if the bullets/numbering would become invisible.

Workaround: Cancel the command in CommandExecuting event:

            this.radRichTextBox.CommandExecuting += (sender, e) =>
            {
                if (e.Command == this.radRichTextBox.Commands.DecrementParagraphLeftIndentCommand)
                {
                    if (this.radRichTextBox.Document.Selection.GetSelectedParagraphs().Any(p => p.IsInList && p.LeftIndent <= 24))
                    {
                        e.Cancel = true;
                    }
                }
            };