At some point, spawners and trial spawners were changed to render their inner faces, but both spawner's bottom face doesn't render unless there isn't a full block face below it, which doesn't seem intentional.
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turns out its not just the bottom face, it's any of the faces. it can even sometimes make the opposite face disappear!
confirmed for 1.21.
[media]

Isn't this completely intentional given that there is a cullface argument in the models to specify this? MC-266463 being triaged supports this.
Relates to MC-269971.