Please refer to the attached sample project and follow the steps from the gif file.
Shrink the ribbon to create sub groups for expanding. Then, maximize the form. You will notice that the groups are still collapsed although there is so much space.
Currently, the possible solution that I can suggest is to refresh the ribbon layout.
sender, EventArgs e)
.richTextEditorRibbonBar1.LayoutMode = RibbonLayout.Simplified;
.richTextEditorRibbonBar1.LayoutMode = RibbonLayout.Default;