Adding to this from my solution search.
if I…
add a ball to the scene,
set it’s Simulate Gravity to FALSE,
play-in-editor,
pause the game,
drag the ball under the parent: making it a child
resume play-in-editor,
The newly added ball follows the parents movement (as it should), and it also have the Sphere component as a child under the DefaultSceneRoot component:
Perhaps, then, it’s the spawning of the ball that is where the error happens.
And since both balls that are behaving correctly have their sphere component as child under DefaultSceneRoot, perhaps that’s what’s causing it?!