Act 3 Viper encounter bug?

In the Act 3 Viper boss encounter, once the boss is defeated, the surrounding soldiers will disappear except a couple of them who will then aggro and attack the player.

But in 0.3.1b, after my newly started character defeated this campaign boss, the soldiers that remained just idled in place, even if I move around.

They didn't attack me until I attack them, and they don't appear to be linked so only the ones I attack will return to normal and engage me, the others remain idling until I attack them.

I'm unsure if this is a bug or a new change to the encounter.