I have updated the hiberate version from 4.0.1.Final to 5.0.7.Final.

Problem:
The hibernate framwork uses hibernate_sequence for the id generation instead of mySequence.

Hibernate configuration:
In order to maintain the backward compatibility, hibernate.id.new_generator_mappings is configured as false value.

Entity configuration:
The id field is annotated with the following JPA API:

	@Id
	@GeneratedValue(strategy = GenerationType.AUTO, generator = "myGenerator")
	@SequenceGenerator(name = "myGenerator", sequenceName = "MY_GENERATOR_SEQ")
	@Column(name="ID")
	private Long id;

DB configuration:
The MY_GENERATOR_SEQ sequence is defined with the "INCREMENT BY 1" option.

The problem occurs for both H2 and Oracle DB.

QUESTION:
I have analyzed the implementation and I don't know if it could be a BUG of Hibernate or any possible bad configuration i did for the backward compatibility???

Analysis:
Hibernate during server startup gets some initialization:
SequenceStyleGenerator.configure method defines the QualifiedName sequenceName as "hibernate_sequence", because it tried to get the value from the Properties params.
At this point I noticed that the MY_GENERATOR_SEQ sequenceName was present into the Properties params at runtime with the "sequence" key (sequence=MY_GENERATOR_SEQ) but the retrieve strategy (determineSequenceName) needs the "sequence_name" as key.
So the key used to set the sequence name into the Properties params is from the deprecated class org.hibernate.id.SequenceGenerator.SEQUENCE because of hibernate.id.new_generator_mappings=false,
but the SequenceStyleGenerator class needs the org.hibernate.id.enhanced.SequenceStyleGenerator.SEQUENCE_PARAM key in order to get "MY_GENERATOR_SEQ".

Potential solutions:
1. set on SequenceGenerator allocationSize=1 (without setting hibernate.id.new_generator_mappings=false)
2. create the sequence MY_GENERATOR_SEQ on the DB with INCREMENT BY 50 (without setting hibernate.id.new_generator_mappings=false)

Both the solutions are not applicable into my context.
So the only applicable "solution" is this one:
Extend the Dialect used for Oracle and H2 and override the getNativeIdentifierGeneratorClass method in this way:

@Override
  public Class getNativeIdentifierGeneratorClass() {
    if ( getIdentityColumnSupport().supportsIdentityColumns() ) {
      return IdentityGenerator.class;
    }
    else {
		//From hibernate-core 5.0.6.Final, org.hibernate.dialect.Dialect
		//the original return value was org.hibernate.id.enhanced.SequenceStyleGenerator.class
      return SequenceGenerator.class; 
    }
  }