This was one of that annoying old design mistakes where Doom and Heretic features were poorly merged together. The Heretic Gargoyle uses very similar coding but performs a subtly different action when actually hitting another actor. This different action was made the default, even for the Lost Soul.
It has now been changed that both monsters use their original action, being distinguished by an actor flag. For compatibility with custom definitions Heretic's behavior, which has been the default in ZDoom will be the preferred one. The one of the Lost Soul can be reactivated by a flag.
