Pick Whip issue

It's not a bug. Simply pointing another layer to a child layer's position never works because the child layer's position after perenting is always relative to comp center and the distance to the parent. 

Three layers, all at comp center:

Parent the Child layer and the Child layer position changes to 0,0 or 0, 0, 0 for a 3D layer. Pickwhip moves the Problem layer to the same comp coordinates as the child layer, but that is not what you want:

Thinking that just subtracting the comp center seems like it might work:

But this fails if you move the child layer:

The solution is Layer Space Transformations. They will lock the problem layer to the child layer.

If you want to be able to move the problem layer and still have it follow the child layer you need to compensate for the comp center offset by subtracting the comp center from the current position value and then add the corrected child position:

Here's the fix all expression that works if the child layer is named "Child":

L = thisComp.layer("Child");
ChildPosition = L.toWorld([0, 0, 0]); // solves problem with 3D layers
// Position Fix
CmpCenter = [thisComp.width, thisComp.height]/2;
value - CmpCenter + ChildPosition

 I have a version of this expression saved as an animation preset. I use it to create virtual child layers that do not respond to position, rotation or scale of a parent layer.