Attached is a simple test case that
1) Deploys process definition with two nodes
2) Starts the process instance that will go to the wait state on first node
3) Starts 20 threads that tries concurrently signal the same process instance
4) The second node writes a record to the database

The test case needs to be executed multiple times to see the incorrect behaviour.
This is an example of run output
Isol 8
Action 1
Action 2
Action 2
Action 2
Action 2
Action 2 1
Action 2 1
Action 2 1
Action 2 1
Signalist 5
Signalist 6
Signalist 8
Signalist 12
Signalist 7
Signalist 13
Signalist 14
Signalist 15
Signalist 9
Signalist 16
Signalist 17
Signalist 18
Signalist 4
Success 7
Failure 13

Explanation of the outcome
4 threads successfully executed the node action including database operation. All database opeartion were comitted (4 new records were created)
3 threads successfully executed the signal operation but no real action was performed
13 threads attempted to execute the signal operation but ended with an exception