About two minutes ago, I was invading Pariden shortly after declaring war. My invasion army was going to meet their forces head-on in their territory. However, I took a one-tile detour to capture their outpost first, then attacked Procipinee's stack the same turn. Said stack was now in *my* territory. Apparently we have reverted to the condition where defeating a sovereign in enemy territory causes that sovereign's perma-death. Anyway, that one battle was enough, and Pariden vanished instantly. 

Frankly, considering that the battlefield was well inside Pariden territory at the beginning of the same turn of the battle, I think this is an exploit and should be prevented.