Single thread execution in camunda 8 self managed

Good evening,
I am locally running camunda 8.6.7 on the self-managed mode, everything works well with the exception of a weird behavior of the execution engine.

I am working on a POC where I have to run some BPMNs containing some workers (service tasks) to be executed in parallel (parallel gateway, and some of them defined by the same “worker-type”).

By the logs, I noticed that the tasks had not been executed in parallel at all.
Furthermore, it seems that all the tasks have been executed by the same thread. Is that possible?

Do you think it is a matter of configuration?
How can I change this behavior to match the process definition as much as possible?

I could not find any discussion related to this topic for camunda 8.
I hope you can help me.
Many thanks