These GFCIs, with the second core, are designed in such a way that they will only sense a short on the (load side neutral to PE (Protective Earth) or EGC ).
The cores are arranged in a way that will not sense, or trip, on the bond at the service directly. Yet as explained below the service bond is essential for this to work.
When I first tested the GFCI I shorted from the line side neutral to PE and the breaker does not trip. When shorting from the load side neutral to PE (at less than 30 ohms) it does trip. This is with no load on the hot lead.
As the linked site explained:
"If there is a N-G short downstream, a current will flow through the N wire, to the G wire via the short, and back to the N wire via the normal connection at the service panel. Since there will be NO similar current in the H wire, this represents a current unbalance and will trip the GFCI in the same manner as the usual H-G short."