To reproduce:
ThemeResolutionService.ApplicationThemeName = "FluentDark";
this.documentTabStrip1.TabStripAlignment = Telerik.WinControls.UI.TabStripAlignment.Left;
this.documentTabStrip1.TabStripTextOrientation = Telerik.WinControls.UI.TabStripTextOrientation.Vertical;
Workaround: this.documentTabStrip1.TabStripElement.ItemBorderAndFillOrientation = Telerik.WinControls.UI.PageViewContentOrientation.Horizontal;