I'm not sure if this is a bug or not and I can no longer test it due to the fact that I don't have /top anymore, but /top would never put me on top of glass. It would send me to the highest non-glass block that was above me. Is there anyway this could be fixed so that it works on glass?
@khobbits should know more about this, but I think it has something to do with glass being a non-opaque block.
