OptaPlanner Kie server service: unable to retrieve best solution

When I try to retrieve best solution from Solver in Kie server OptaPlanner extension, JAXB and XSTREAM works correctly but JSON doesn't. JSON marshalling isn't able to map planning variable in planning entity.

It can be simulated using cloudbalance problem which is part of Kie server OptaPlanner integration tests. See test method testGetBestSolution in OptaplannerIntegrationTest.