I'm upgrading from Spring 4.3.23.RELEASE to Spring 5.3.20 and running into issues with the save and flush of the record due to that Spring is not using the correct next available key value from the Oracle SEQUENCE that is configured in the Java domain object. Seems it is picking some arbitrary key value that was already used in the Oracle table instead of the next value in the Oracle SEQUENCE object. I'm guessing there is some incompatibility possibly between the spring-data-jpa (2.6.5) and the hibernate-jpa-2.1-api (version 1.0.0-Final) that is causing the execution to misbehave.
I tried switching the generation strategy from 'AUTO' to 'SEQUENCE' and making the domain class pk attribute type from long to Long which made matters worse.