That's not a bug, column render is a complete renderer of the cell means it renders the cell the way you write it.
You are overwriting the inbuilt renderers of the grid by implementing yours.
If you want to skip your renderer for the group titles and summary rows, then you can do so by checking pq_gtitle and pq_gsummary properties in the rowData and return nothing when true.
if ( ui.rowData.pq_gtitle || ui.rowData.pq_gsummary ){
return;
}
else{
return your custom renderer
}