Some times it accesses the current staging list, instead of the one in the segment memory. The NotNode should use the passed staging list, and not retrieve it directly. They should always use takeAll, to ensure they are thread safe.
Fixed by https://github.com/droolsjbpm/drools/commit/eca450ead9250116d2822b57eeae5cd2449a638e
Verified that the fix is present in ER5.