See this jsfiddle:
If you move the blue box at the bottom into the child droppables (green boxes) then the green boxes will turn yellow.
If you move the blue box from bottom to top, then the parent will also become yellow.
But if you move the blue box from top to bottom, then the parent stays red.
The correct behavior is that the parent has to stay red regardless of moving the blue box from top to bottom or bottom to top.
I've been analyzing this for two days, and I'm sure that this is a bug. This is caused by calling the parentInstance._out and parentInstance._over in the same $.each loop in $.ui.ddmanager.drag.
If you move the blue box from bottom to top, then the parentInstance._out will be called when the blue box goes into the box with a lower index (above the green box that the blue box left) in the $.ui.ddmanager.droppables. This means that the parent droppable is going into a greedy child.
Then when the loop reaches the index of the droppable that is left by the blue box (the box below the then hovered box) the parentInstance._over will be called because we're moving out of a greedy child.
In short, this bug causes the parent droppable going into and out of its nested greedy droppable in the same $.each loop.
I've made an un-optimized fix for this, but it should give the general idea of how to fix this behavior.
This behavior should not happen as it causes a nested greedy droppable
This jsfiddle contains the patch for $.ui.ddmanager.drag method: