That is a great analysis Jack and kind of something I've been kinda shush about.
I have a buddy who also had a pup that refused water. She was fed kibble though, the only way he could get her to drink was to pour it out of the bowl onto the floor. She was about the same age but only shipped a few hours away from where she was born. His thought was if she's to dumb to drink life sustaining fluid I don't need her. She was becoming dehydrated so he felt it was best to euthanize.
Looking back at that situation and yours could it be something as simple as not having the competition of her littermates to drink? I know when I've had a small litter 1,2 or 3 pups they mature much slower. But a larger litter they are all trying to be 1st at everything, and if you put down a bowl of water they are trying to out drink the littermates! So like me on a Saturday night maybe she needs a drinking buddy to show her the ropes? lol
Or maybe the bowl you have is to reflective or moves and makes a sound and it spooks her?
Good luck and let us know what happens.
S_B