PGIA Gallery active thumb overlaps navbar

When scrolling a pgia gallery with thumbs over the navigation bar of a bootstrap 4 onepage site (blueprint with pgia animated sections) the current thumb overlaps the bar. It has z-index:1, but that doesn’t seem to matter – even with the navbar z-index set to 1000 or more nothing changes that.

Why is it? I can’t see whats going on. Can someone here?