My brother has been learning Defold and I was shocked to hear from him that nodes on the same layer were able to be sorted by z! That sounded really useful.
But unfortunately it only works that way in editor. In engine they still sort based on order in the tree. So probably a bug in editor 2.
While testing it I also noticed that if you set the z position of a node outside of render script z distance that nodes also stop drawing. This seems like a bug in the engine as well.