I was curious to see if I could replicate this and it turns out that I can, quite easily.

Zombies that are set to passive behavior will NEVER attack, even when specifically directed to do so via command zombie attack <target>. They won't even advance on the target. Curiously, if you let your zombie begin to attack something while it has either defensive or aggressive behavior, and then switch it over to passive behavior it will go back and forth between retreating and pouncing to melee with the target, but it'll never attack. That's assuming you have it set to offensive stance for the pounce, of course.

This definitely seems like a departure from how it previously functioned. I seem to recall even passive zombies could be made to attack a target when specifically told to. This description I found on elanthipedia suggests that as well.

>>BEHAVIOR options mirror an Empaths Guardian Spirit - AGGRESSIVE will auto advance on and attack all monsters, DEFENSIVE will only attack something that engages it first, and PASSIVE will wait for the Necromancer to issue a command.