After we switched OptaPlanner to use executable model for DRL, we started seeing some weird behavior. We reproduce it in the following test:

https://github.com/triceo/optaplanner/blob/incremental-bug/optaplanner-examples/src/test/java/org/optaplanner/examples/travelingtournament/app/DroolsReproducerTest.java

It would be too hard to extract the test from OptaPlanner entirely, but we have managed to minimize it to insert/update operations. The actual Drools code starts here:

https://github.com/triceo/optaplanner/blob/incremental-bug/optaplanner-examples/src/test/java/org/optaplanner/examples/travelingtournament/app/DroolsReproducerTest.java#L118

The bug, then, happens here:

https://github.com/triceo/optaplanner/blob/incremental-bug/optaplanner-examples/src/test/java/org/optaplanner/examples/travelingtournament/app/DroolsReproducerTest.java#L143

The DRL we use is here:

https://github.com/triceo/optaplanner/blob/incremental-bug/optaplanner-examples/src/test/resources/org/optaplanner/examples/travelingtournament/solver/reproducer.drl

Please read the code comments that will explain the key points.

Do note that this must be some corner case, as most of OptaPlanner Examples continue to work with exec model enabled.
Should more information be needed, please contact jlocker in my absence.