Tom - One of the concerns of conservators of collections of wood turned objects is that CA glue is turning out not to be archival. CA glue gets progressively more brittle over time and does not handle shocks well as it gets old. Most of the ages I have heard about are decadal.
The failure of the glue joint between the tube and the casein shell is clear. What cannot be seen in the photos is where the failure is occurring. It could be in the glue, at the surface of the casein or at the surface of the tube.
Casein is a tricky material to work with and has many known weaknesses. One study on ways to improve its performance describes the objectives "..... in order to improve the film-forming defects of casein such as hard, brittle and excessive moisture absorption performance, etc."
Under high magnification you may well find well find that failure is at the casein - glue interface.
Any chance that the pen/box was dropped or jarred hard??