I've seen this happen when there's a walkmesh error.
It's also possible to make an elevated walkmesh, of course, to create the illusion of a creature floating in the air or standing on a placeable. Normally that would be a fixed feature of a tile - I remember seeing some experiments with placeable walkmesh, but I'm not sure whether they ironed out all the issues with that.
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[Thank you for sharing, friend @Knight_Shield !]
(it was on a thin bridge over an abyss so I could see under it)
It's also possible to make an elevated walkmesh, of course, to create the illusion of a creature floating in the air or standing on a placeable. Normally that would be a fixed feature of a tile - I remember seeing some experiments with placeable walkmesh, but I'm not sure whether they ironed out all the issues with that.