Right at the start of the first mission I see an enemy standing near a fire, not moving. It doesn't become active until I've moved around a bit.
Same happens at the 1st checkpoint. An enemy is standing next to the cube weapon. Doesn't wake up until the door closes, which only hap[pens after I cross the room.

That's normal.

As FulVal says, it is normal, these enemies are named Burned Ones and they have to be a volcano victims caught in the blast, just waste them when they wake up, definitely not a bug.