That's another, albeit related issue. The sorting has to use some point of the object to determine its depth. So it takes the center of the object transformed into camera space. As you can easily imagine, this sorting can never be perfect in all cases (if it where, nobody would need a depth buffer at all), which leads to these results. For special cases, you can tweak the sorting with Object3D.setSortOffset(), but that's most likely not suitable here.