After Effects 2020 shape layer path ignoring 3D orientation when used to parent a null?
The shape layer's path is in layer space. In other words the coordinates are relative to the layer, regardless of where the layer is in composition space. The contents of a layer are always at 0 on the z axis relative to that layer. So when you apply that path as the motion path to a layer in composition space, it will always move on the X/Y plane, at 0 on the z Axis.
To make the null layer use the same coordinate space as the shape layer, you can simply make it a child of the shape layer. Then the coordinates will be relative to its parent. If this is not possible, you could use the toWorld() layer space transform in an expression, thus:
thisComp.layer(&quot;shape Layer&quot;).toWorld(value);

and apply that to the position. What that does is take the position value and find out where that would be in the layer &quot;shape layer&quot;, then give the value of where that is in comp coordinates.
There's a little gotcha, in that paths on shape layers are measured in comp space. Before copying the path to the null, you might want to set the anchor point of the shape layer to the center of the content, using ctrlalthome. Otherwise you can subtract its position from the value of the copied path:
thisComp.layer(&quot;Shape Layer 1&quot;).toWorld(value-thisComp.layer(&quot;Shape Layer 1&quot;).transform.position)