This isn't something I'm aware of. Could you log this with Support to investigate it further with you?
Thanks for getting back to me.
After having a look into this, it seemed that the task process assigned to the 'Add Task' automatic action was set to blank when the task assignment was updated in the request process (it seems when you make a change to a task in the main process, the process linked with the task is lost). The task now has the correct process linked and is working correctly.