If you ever wondered why during polymodeling in zbrush you keep selecting several polygroups by CTRL+Shift clicking on only one:
Zbrush for most of brushes and selections uses vertices. And if your polygon doesn’t have any other polygon in same polygroup next to vertex that you clicked – it just selects also next polygoup.

So in this case you can use Select Lasso tool and click on one edge to hide full polygon loop. And then invert the visibility by CTRL+Shift dragging outside of mesh: