Hi Maciej,
I'm Namjun who work with Tim and Khetha. What we found was that when we remove the timer task(10min) from our workflow it works fine and workItemByteArray gets updated properly. Originally, we have a timer task just right after begining(start event) in our jBPM workflow.
Do you have any idea if there is any relation between timer task and storing workItemByteArray?
Namjun