If she's only seen in cutaways they might not have even had her on set at the same time as the rest of the scenes were shot. She might've been first-in or last-in.
Good point. In fact I think the only time we ever see human Isis is when she's sitting on the couch and there's no one else in the scene. So she may not have even met the rest of the cast.
And at the time, IIRC, extras had their own union (they weren't yet SAG members) and so were almost never credited.