Consistency level and graph.addVertex() 

A consistency level of ONE can cause inconsistent behavior during vertex creation.

The DataStax Enterprise Help Center also provides troubleshooting information.

Vertex id allocation requires QUORUM consistency to ensure that allocated ids are unique. Vertex ids are allocated in blocks. If a block runs out, write failures can occur and a consistency level of QUORUM cannot be met. Graph clusters can operate while falling below QUORUM, but it is possible for queries involving adding a vertex can fail if a consistency level of ONE is used.

Do not use a consistency level of ONE for operations that add vertex data.