This may be a change in “design” ut not easily identified in the change logs - quoting https://docs.jboss.org/hibernate/orm/5.0/userguide/html_single/Hibernate_User_Guide.html#identifiers-generators :

The discussions below assume that the application is using Hibernate’s "new generator mappings" as indicated by the hibernate.id.new_generator_mappings setting or MetadataBuilder.enableNewIdentifierGeneratorSupport method during bootstrap. Starting with Hibernate 5, this is set to true by default. If applications set this to false the resolutions discussed here will be very different. The rest of the discussion here assumes this setting is enabled (true).