The caption text of the RibbonBarGroups disappears when switching themes runtime. The important part is that this behavior is observable when switching from the Office2010Blue theme to other themes. In this specific theme, the FillPrimitive holding the TextPrimitive (caption text) has the PositionOffset property set to 0,-1. This minus one pixel is messing up the layout during theme change.

What can be done as a workaround is to reset this property for every group. When the Office2010Blue theme is applied run this code:

private void radButton1_Click(object sender, EventArgs e)
{
    Office2010BlueTheme theme = new Office2010BlueTheme();
    ThemeResolutionService.ApplicationThemeName = "Office2010Blue";
    foreach (var item in this.ribbonTab1.Items)
    {
        if (item is RadRibbonBarGroup)
        {
            var group = item as RadRibbonBarGroup;
            group.Children[1].Children[0].PositionOffset = new SizeF(0, 0);
        }
    }
}