Graves bug?

Alright I experienced this earlier today, but I'm ill atm and wasn't thinking about it till just now, but it seems that at certain points in the game graves shotgun WILL NOT damage yorick's ghouls. They're still getting pushed back and if I recall correctly absorbing bullets. Range didn't seem to matter and about 5 minutes in or so it appeared I could once again damage the ghouls. I apologize for not saving the replay. Can anyone else confirm this?