Finally solved the issue.
I couldn't locate the problem, but I rebuilt the entire UIButton structure (where the problem was coming from), with the exact same properties and I can now move the Z. It seems the prefab had somehow locked it's Z on me, although I'm not entirely sure how.
I have gone back to the same project and actually found that somewhere in the hierarchy the z was set to a scale of 0, preventing an transforms that were children from moving along the z and causing issues with separate atlases being unable to define which to draw on top of which. This wasn't in the same place I was having that issue but I assume this was the issue I ran into.
Just wanted to come back and post what I think was the solution.
