First thing to notice is that if you are using 2 different persistence units, then you can clean up 'org.jbpm.persistence.jpa' by removing all the 'org.jbpm.task.*' classes.
The dependency you need to have is:
<groupId>org.jbpm</groupId>
<artifactId>jbpm-persistence-jpa</artifactId>
Other than that, make sure you are actually using 'org.jbpm.persistence.jpa' persistence unit in your application.
Best Regards,