I have an issue. Currently I have one grid with column group, I differentiate the column group by color (define in CSS) when I resize the one of the column or resize the window browser, the last column group and normal column background color automatically change to the default color.
I have tested the example that you have sent but could not reproduce such issue.
Could you provide some more information? Do you have column reorder enabled? Have you inspected the styles of the column which color changes?