I believe that raid 'leftovers' who don't get killed during an event, do remain spawned and create havoc in the biomes until they die, since biome vs biome is happening...causing "out of biome" NPCs. I see fulings all the time in the neighboring Mistlands biome and Black Forest NPCs there as well, it's at an intersection of 3 biomes.

My observation: during a Night 2 Day cycle, the night spawns disappear when dawn arrives, while after you get the prompt for an event ending.....they do not 'poof'/despawn out of existence/instancing.