Current OptaPlanner's persist-jpa uses Hibernate ver. 5.1.15.Final, but spring-boot-jpa ver. 2.x uses Hibernate ver. 5.2.x.
One effect of the upgrade is the parameter type in

org.hibernate.usertype.CompositeUserType

interface changed from

org.hibernate.engine.spi.SessionImplementor

to

org.hibernate.engine.spi.SharedSessionContractImplementor

which makes

org.optaplanner.persistence.jpa.impl.score.AbstractScoreHibernateType

class and all its subclasses fail to implement the method in the interface.

The discussion may be foud here