Think LeBashar is right. From the source code - the tank armor fits the 1x1 criteria (along with the rest) so it becomes a zombie. Maybe there needs a Armor or Unit class rule such a "Biological", "Mechanical", etc.
ExplosionBState::explode()
...
if (!_item->getRules()->getZombieUnit().empty()
&& victim
&& victim->getArmor()->getSize() == 1
&& victim->getSpawnUnit().empty()
&& victim->getOriginalFaction() != FACTION_HOSTILE)
{
https:// converts the victim to a zombie on death
victim->setSpecialAbility(SPECAB_RESPAWN);
victim->setSpawnUnit(_item->getRules()->getZombieUnit());
}