UPDATE: So I replayed from a new save all the way to day 16, this time breaking all the mannequins I could find before they teleport. They similarly spawned in a pile in one location, but there were in fact a lot less of them and there was not a visible mannequin floating in the air this time. This leads me to assume either what you've said is true, and it's SUPPOSED to be in the air, and I've now encountered a bug where that doesn't happen, or that what I found originally IS a bug, and getting rid of extra mannequins removed the floating one.