Signal Boundary Event does not work at second time

A signal boundary event on an embedded subprocess does not work anymore after the subprocess has been previously completed regularly (not via the boundary event).

In more detail ...
Test 1: This one works:

• Exiting sub process via boundary event.
• Exiting sub process regularly

Test 2: This one does not work:

• Exiting sub process regularly
• Exiting sub process via boundary event.

I realized, that recreating a fresh knowledge base for every process execution seems to be a workaround.

The issue must be related to the fact, that JBPM inserts NULL as "element" into the EVENTTYPES table when the problem occurs.

Please find a maven project with a test case and a test process attached to this JIRA.