Child Frame doesn't scale with autoscaling parent Frame when ScrollingFrame is a child of it?

I’m having an issue with a frame not stretching with its parent with AutomaticSize enabled. My goal is to have a shadow effect behind my window. It works if I use a separate Frame to automatically resize the parent and the shadow, not when I use a ScrollingFrame.

Here shows the problem. I also set the ZIndex of the shadow to be in front of the main window:


Window hierarchy:

Any ideas?