Unexpected behavior from Drools' collectSet and difficulty effecting undo moves - drools

Question

What I actually wanted to do is to have a Drools rule as so:
rule "globalRequiredPredecessorAfterMe"
when
$rpAll: Set(size>1) from accumulate (
Customer(vehicle!= null, vehicle.vehicleTyp != VehicleTyp.DUMMY, $rpAfterMe: requiredPredecessorsAfterMe);
collectSet($rpAfterMe)
)
then
scoreHolder.addMediumConstraintMatch(kcontext, - $rpAll.size()-1);
end
Unfortunately, on the second move in CH drools awards me with a
Exception in thread "main" java.lang.RuntimeException: java.lang.NullPointerException
at org.drools.core.rule.SingleAccumulate.reverse(SingleAccumulate.java:124)
...
Is this a feature?
I swallowed my pride and to wrote a fantastically complex Listener which is supposed to to what the Drools rule above did. With an assertionScoreDirectorFactory (an EasyScoreCalculator) I get score corruption. On a closer look, I get a corruption in one of the two cases:
My planning entity's previousXXX is set exactly to the same value
An Undo move is done
To investigate, I set the solution to the state just before the score corruption occurs. I use the CreateChangeMoves from Optaplanner's 7.4.1.Final SolutionBusiness class. (On a side note, it would be really helpful if one could also set a MoveCountLimit in the SolverConfig.)
So I create the scoreDirector as follows
SolverFactory<MySolution> solverFactory = SolverFactory.createFromXmlResource(
SolverConfigXML);
solver = solverFactory.buildSolver();
ScoreDirectorFactory<MySolution> scoreDirectorFactory = solver.getScoreDirectorFactory();
scoreDirector = scoreDirectorFactory.buildScoreDirector();
scoreDirector.setWorkingSolution(unsolvedSolution);
within a JUnit 5 test and then perform several ChangeMoves to mimick the solution just before the offending move occurs. However, when I perform the offending move as in bullet 1 above,
// customer SUK0002030's previousVehicleOrCustomer is DUMMY_3
cm = createChangeMove(nameToCustomerMap.get("SUK0002030"), "previousVehicleOrCustomer", nameToVehicleMap.get("DUMMY_3"));
cm.doMove(scoreDirector);
I get a
java.lang.IllegalStateException: The entity (SUK0002030) has a variable (previousVehicleOrCustomer) with value (DUMMY_3) which has a sourceVariableName variable (nextCustomer) with a value (null) which is not that entity.
Verify the consistency of your input problem for that sourceVariableName variable.
at org.optaplanner.core.impl.domain.variable.inverserelation.SingletonInverseVariableListener.retract(SingletonInverseVariableListener.java:87)
...
When I set the previousVehicleOrCustomer to null, and then back to DUMMY_3, everything is fine and no score corruption occurs.
Similarly, when attempting to create an (offending) UndoMove of the previous Move cm, bullet 2 above, as so
cm.createUndoMove(scoreDirector).doMove(scoreDirector)
I get the same message:
java.lang.IllegalStateException: The entity (SUK0002014) has a variable (previousVehicleOrCustomer) with value (Vehicle_0) which has a sourceVariableName variable (nextCustomer) with a value (null) which is not that entity.
Verify the consistency of your input problem for that sourceVariableName variable.
at org.optaplanner.core.impl.domain.variable.inverserelation.SingletonInverseVariableListener.retract(SingletonInverseVariableListener.java:87)
...
Of course, if I manually create the UndoMove (by simply setting the previousVehicleOrCustomer back to null), everything is fine.
These moves really should be possible and I'm really curious to find out what's wrong.