Are you sure that your passive firewall is actually passive and not processing traffic. Just because this unit reports that it's staying in active after the split-brain event doesn't mean necessary that the peer firewall didn't come to the same conclusion. During the 428s where you were in a split-brain event however you would definately have both firewalls processing traffic.
Are the sessions routing to the non-existing IP possibly be tied to existing sessions? Depending on your policy setup it would continue to take that path, or attempt to. If this is tied to specific sources or destinations you could try clearing the session list with a filter for that criteria and see if that clears things up.
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