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Monsters who are spawned on 0 height 3d floors, either translucent or opaque, will appear to fall through, their sprite will move between the levels, possibly when a state change happens (monsters will finish their attacks).
This happens in both software and opengl, and it happens whether they were placed there in the map or spawned later on. Monsters spawned on 3d floors with a height of 8 do not have their sprites fall through.