Actually, it was the quail calculator thread, I mistyped in the OP:
https://www.backyardchickens.com/threads/the-confusing-quail-calculator.429645/
Specifically from post #8: "Yes, white genes can be lethal. There is a "light down lethal" and they are English Whites but in 3 weeks they die or don't make it as little chicks. Some make it but carry the gene."
But in all my other googling, I could only find references to the golden color in quail having a lethal issue.
Either way, I do think you're correct that in this particular case, it's just a failure to thrive. She's not a hearty chick. I'm selfishly hoping she continues to be happy enough to make it to full size, but I'm prepared to cull her earlier if it looks like her quality of life changes for the worse.