Brett Meyer commented on an issue
Hibernate ORM / Bug HHH-7361
aggressive release closes JDBC connection while still in Loader.loadEntity()
JDBC auto-commit causes "result set is closed" exception on rs.next() in Loader.

Nested loads (proxy -> Map collection using entities as keys & value, in this case) may open a second PreparedStatement.

FireBird (JayBird 2.1.6) driver closes existing open stmts/resultset, when 2nd nested load prepares it's statement.