@Christian but then Hibernate isn't fullfilling JPA 2.1 spec. section 4.6.17.3 (Invocation of Predefined and User-defined Database Functions). There it says FUNCTION will invoke those database specific ones. And GROUP_CONTACT exists in HSQLDB, I checked that using a JPA Native query.

I insist that this is working in Payara using the same DBMS (HSQLDB). So it's an Hibernate error not implementing well FUNCTION for any existent DB function (GROUP_CONCAT in this case for a reason I don't understand, as it's working with other HSQL functions).