Incremental compilation requires to provide the list of the modified files since the last compilation. However this could be error-prone and violates the DRY principle, because this list could be automatically calculated.
Fixed by https://github.com/droolsjbpm/droolsjbpm-knowledge/commit/a6983db23 and https://github.com/droolsjbpm/drools/commit/8040e5c15
Mario, please go ahead and commit to 6.0.x.
Both commits are in product repository. Since the incremental build is internal feature and test for this fix was provided in community, I mark this issue as verified.