I'm currently using R2 2018 SP1 and I'm experiencing a display issue with the grouping header. In my project, I have two grids that are very similar. Both use grouping but one one grid has static headers.
When I use static headers, the grouping header show the column grid lines (i.e. row renders a <TD> for each column). But when I just simply remove theUseStaticHeaders="true" attribute, the grouping row is displayed as expected without the column grid lines (i.e. row renders with as one TD and a colspan attribute). Also note that I'm using the WebBlue theme.
See the attached screenshots displaying both situations.
I was unable to find this specific situation in the current grid demo so I was unable to confirm if this is a problem in the current release.