She didn't have to weep.
There are more ways to emote than weeping and staring blank faced. Disbelieving anger would have been a solid choice. Screaming, wide eyes, heavy breathing, shaking of the head, there are several things an actress can do to "react" to something terrible.
And in either event, breaking her persona for the death of her child would actually be a powerful character moment and sell how meaningful the death is to her. If she's always on point, always confident, always controlled, and this ONE moment breaks her, it makes the moment even bigger.
Instead she about as sad as she ever looks ,honestly less sad than other moments, which undercuts the significance of the moment imo.