When trying to start 45 process instances which contain an asynchronous service task with of class ShellActvityBehavior (with the class field wait=false), some of them don't start, even though they show up in the diagram, as seen in the image:
As you can see, the execution's current activity is a receive tasks that waits for the long running service task to end. When it ends, it sends a message that the boundary event catches and the execution flows to the "Quick operation".
In reality, the long running task didn't start. I know this because the task writes start and end messages to a log, with its process instance ID. That log has no indication of that specific task (of process instance 1197546).
Some general information:
I'm using activiti 5.18.
The process instances are being started via the java API.
This is how I build the process engine (Could altering the configuration solve this?):