[1.2] If I declare war on an enemy, their undefended pioneers should run away

I caught Warlord Verga out roaming the wastes with only a 3-man spear unit, so I took the opportunity to declare war on him while we were both on neutral ground.  My sov and his two 3-man spear units defeated him easily and 4 turns later, a Yithril pioneer moved directly toward my army.  I watched it move toward me, as i was making for a lost library and NOT chasing the pioneer.  The pioneer could have vanished into the FOW but blithely continued towards me.  

If only my sov had silver tongue this game!