Thanks for letting me know about the grenadiers. Short version - it's because some of them are running the wrong behavior tree that causes them to try and fire 'automatically' like another enemy does that doesn't appear in the demo. But they lack the animations for doing it properly so they just turn invisible instead.
The doors are a perennial problem child and will be fixed for sure in the next update - which will (finally) have the first episode's complete OST.

