This is the left side dual of the issue reported https://bugzilla.redhat.com/show_bug.cgi?id=1056599
Fixed by https://github.com/droolsjbpm/drools/commit/bb57cb66deae757e7db2adb3d53af52f30f78b14
Verified that the fix is present in BRMS 6.0.1 ER1 and the corresponding test passes.