ZDepth passes ignore transparency (so far as I've tested in Vue for example - I dunno if Poser uses ZDepth the same way)  They only give depth data, something that is transparent but the same "depth" as something opaque should have the same greyscale value .

(unless I'm mistaking what you mean of course)

Here's what I mean.
This is the actual render - back 2 cubes are at the same distance from the camera

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