Not sure if this is a bug. But what I was expecting was the same thing that happens when you SetScale() on a frame that has children.
What happens instead is every child texture or frame and the parts of the backdrop borders get scaled. Looks ugly. As a work around. I had to do an OnUpdate that applies SetScale() based on Animation:GetProgress()
Update:
Bad Scaling
https://www.youtube.com/watch?v=2CpTr0187I4
Good Scaling
https://www.youtube.com/watch?v=d1dQ6idHK0s