If the child you are adding is of large size then add child to scroll box will fail without any errors. If you can wrap the widget with proper size box then it will work. Try that.
This thread is old, but my solution was to change the anchor of the child widget. For some reason using the full screen anchor allows the widget to be collapsed. I guess its trying to stretch to 0,0,0,0
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BRUH THE ANCHOR LOL that was it for me… in 2020 T_T ue4.24