Could the Metabones not be quite straight? In addition to the floating lens elements (which could cause the strange behavior, especially in cases where two or more groups of elements move by different amounts), a slightly tilted adapter (as you say, tenths of a millimeter could screw this up) could introduce odd effects - especially if the tilt of the adapter is at a strange angle to the tilt you are deliberately introducing to the lens - there's no reason to assume the adapter's tilted only parallel or perpendicular to the lens tilt axis - it's just as likely to be at 37.963 degrees off axis (or any other random number).
In the world of (related) optical exotica, there were at least a couple of 1990s Zeiss lenses for Hasselblad that had separately adjustable floating groups. Almost always, a lens that has two or more groups moving has them cammed together in some way, so there's only one focus ring. These oddities (at least the 40mm and 50mm CF FLE and various updates) used two focusing rings - the main ring moved all the elements, while a separate FLE ring moved one group in relation to the others.