Quote - ... but this seems to have been ignored in the Z Depth render

Unless the transparency for a pixel is 100% (i.e. completely invisible) then I don't know what depth you would want to have for it other than that of the object at that point.

Put another way, I'm pretty sure this behaviour is correct, as having transparency work the same way for depth as it does for colour wouldn't make any sense (the calculated depth would be somewhere between the front object and whatever was behind it).