Activation group doesn't work as expected

In some rare cases activation group doesn't work as expected. For example, two rules with same ruleflow-group and activation-group but different salience are both get fired if reuse kie session.

Some investigation notes to help address.
1) This issue never happen if use a new fresh kie session.
2) I also tried to write a drools style unit test (testActivationGroupIssue in ExecutionFlowControlTest.java) but can't reproduce this issue.