It still doesn't make sense, though. With the tank pumps running and the manifold(s) pressurised, fuel is going to leak
out rather than air leak
in... Unless there was a massive failure of NRVs leading to a kind of short circuit in the fuel supply... But even then, the fuel in the pipes is still at a higher pressure (atmospheric plus a bit of gravity feed) than that at the entry to the engine pumps.
Still pointing towards some sort of restriction, I feel.