I verified that this bug is not exclusive to mass elements attached to beams. It seems every constraint/load/boundary/connection to the nodes of a quadratic beam element is applied only to the outer nodes of the expanded face, as reported by Victor in the issues of the source code at GitHub. It is indeed a bug in the CalculiX nodes expansion for beams. Maybe by using linear elements, this could be avoided, but many other problems could arise due to the limitations of these kinds of elements.
