Drools Planner score corruption during construction heuristics if there are rules that check if planning variable is null

rule "no foo without bar"
    when
        $foo : Foo(bar == null)
    then
        insertLogical( new IntConstraintOccurrence("foo without bar", ConstraintType.NEGATIVE_HARD, 1, $foo) );
end

...
        if (assertUndoMoveIsUncorrupted) {
            GreedyFitSolverPhaseScope greedyFitSolverPhaseScope = moveScope.getGreedyFitStepScope()
                    .getGreedyFitSolverPhaseScope();
            Score undoScore = greedyFitSolverPhaseScope.calculateScore();
            Score lastCompletedStepScore = greedyFitSolverPhaseScope.getLastCompletedStepScope().getScore();
            if (!undoScore.equals(lastCompletedStepScore)) { //<-- HERE scores deviate!
...