See the bubble on the left -- the plant inside the bubble appears behind the green plant.
Related to polygon IDs, not Z, afaict. I saw the same bug while implementing polygon IDs in melonDS. I don't remember the exact cause, but basically: the polygon ID check for translucent pixels is only done when the destination pixel was translucent too (regardless of the stored alpha), except when drawing shadows.
(this, based on what I observed and tested. may be wrong)
I implemented that part of polygon ID stuff already. It was, for example, causing trees to show through each other in SM64 before I fixed it. Wonder if I got it wrong somehow.
DS GX: Retain translucent polygon ID when drawing opaque fragments (f…