Load a model, close it and load a new model in ModelBuilder would track the wrong model manager.
Bug: Load a model into Modelbuilder. you can get bitflags for each cell. Close it then load a different model. Boom! You cannot get any bitflags for any cell in the second model... Because the current manager cannot find this entity. link
Inspection(Thanks to David): CMB is creating a new vtkSMModelManagerProxy (which is what owns the client's smtk::model::Manager) but that some part of CMB is holding on to the old smtk::model::Manager and trying to use it to fetch information about the new model entities.