jBPM 5.4 seems to have some issues with
drools-compiler-5.5.0.Final.jar
drools-core-5.5.0.Final.jar
drools-persistence-jpa-5.5.0.Final.jar
When you have multiple sessions - its triggering timer tasks repeatedly & some timer tasks even miss.
If you replace these three jars with jBPM 5.3 ones - then it looks ok.