To reproduce: please refer to the attached screenshot

        public RadForm1()
        {
            InitializeComponent();

            ThemeResolutionService.ApplicationThemeName = "Fluent";

            this.BackColor = Color.White;
            this.radGridView1.EnableGrouping = false;
        }

Workaround: this.radGridView1.TableElement.Margin = new Padding(0, 1, 0, 0);