I don't know if it is simply undocumented, but when you put a player start above a floor that is made of a static mesh it attaches itself to that static mesh's collision. So watch the collision.
This is intentional behavior as it is a way to check that a player start is based on something and isn't floating in the middle of no where. You also notice a similar behavior with path nodes (or anything subclassed from navigation points). This also occurs on BSP.
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