Hello,
This happens because of the line
<property name="hibernate.hbm2ddl.auto" value="create" /> found in persistence.xml and hibernate.cfg.xml files.
Basically what this does is to recreate the tables, thus data gets lost.
The persistence related files are in META-INF of your task service, also in jbpm-gwt-console-server.war\WEB-INF\classes and jbpm-gwt-console-server.war\WEB-INF\classes\META-INF of your console server.
You can simply comment it out or read more about it... check out the following thread and the links mentioned,
https://community.jboss.org/message/641541#641541