Thanks for the responses.
So, it doesn't have a real string on it yet, just tarred marline. Not sure if that affects anything, but I don't remember it bothering anything before.
So it could be weak or stiff spine.
I brought it home and shot one of my 55-60 spine 32" arrows from it and it flew nice and straight. So, my original thought is that the youth arrow is too weak. Now, the youth arrow is 24 1/2" and actually feels pretty stiff relative to my arrows, which are a lot longer. I don't have a spine tester, they might be too short to fit on one, right? So, maybe it's relative spine is too stiff?
Confusing. Luckily, I saw an arrow fly straight, so it's just a matter of getting or building the right arrows for this bow. The youth arrows will have to be passed down to a smaller cousin and I guess I'll have to make another bow to go with them!