Flak detonating regardless of targeting settings

I have a standard missile with a 5kg flak warhead, set to only target ships. I was testing a strategy against it, firing a counter missile (nuke), but the incoming flak missiles actually go off when the counter missile gets close, even though it's marked to detonate only when targeting ships. The entire flak salvo is thus detonated and lost, even before the nukes go off.

I even tried unchecking both targeting boxes on the module screen, to no avail. The flak payloads still go off regardless.

My flak missile is then useless, just a dummy missile thrown against it kills the entire salvo.