(Possible ) bug when multi-sphere and periodic boundary are used together

I've observed some strange behaviors of multi-sphere particle motions when periodic boundaries are used.

For example, when I inserted multi-sphere particles (with gravity) into a hopper, and assign periodic boundaries in the direction normal to the cross-section, all particles are supposed to drop down due to gravity. But lots of particles strangely appear on the very top of the vertical boundary after a while, and they never move! (see the attached figure).

So I just want to confirm if there is similar user experience that has trouble to use multi-spheres and periodic boundaries together?