Hm, sounds like a bug.

But transparent PNG background definitely works in PP2012 - I just saved an image out.

Just for the info of others: Render over ___ (colour) isn't about what colour the background will be with a PNG (although it should be with a JPEG). It's info Poser needs in order to anti-alias against the colour of background you want. If you have a transparent PNG rendered over black, it will look funny if you put it over a white background, and vice versa. Or at least, as I understand it, that is the intention. Rendering over the correct background colour will help you avoid "jaggies" when you put the PNG to use.

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Hardware: AMD Ryzen 9 3900X/MSI MAG570 Tomahawk X570/Zotac Geforce GTX 1650 Super 4GB/32GB OLOy RAM

Software: Windows 10 Professional/Poser Pro 11/Photoshop/Postworkshop 3