Hey @MirekSz,
Your question comes in the right time. This is actually is a problem for multi instance executions since the beginning of the project and is a problem for all of the similar BPMN engines out there.
The good thing is that starting from Flowable 6.6.1 this will no longer be a problem and it would be extremely easy to perform variable aggregation of multi instances into one single array node.
We are planning to write some blog posts about this and explaining how you can use it. For now you can have a look at the PR where this functionality was added.
In order to make it work now you will have to use a multi instance sub process with a service task after the CallActivity, store the output variables in the execution locally, then in the service task add them to an array node on the execution. You’ll have to use a sequential multi instance for this to work properly.
Cheers,
Filip