What I understand from the DMN standard, the DRG stands for the graph of dependencies of decisions. The nodes in the graph are the decisions, the edges are the requirements. A DRD is a view of a subset of that graph. With this in mind, I would say that this is the invariant:

• A decision that is present in at least one DRD is part of the graph. 

In Kogito, this seems to be implemented in a halfway manner. If a decision is added in a DRD, it doesn't show up in the DRG. However, if a decision is removed from the DRG, it is removed from all DRDs as well. 

I would expect the other flashpoint of the invariant should be implemented as well: If a decision is added to a DRD, it should become part of the DRG.