There is no task in UWL when i assign expression as responsible for a task

Hi @,

I create a simple process BPM on Process Composer, there is a task and a service to start this process.

When i assign specify user for this task (e.x : ABC), i start process using the service, it's ok, there is a task is assigned to ABC.

But when i use expression to assign for the task, the value of this expression will be mapped with data i input when start process using the started service. There is no task on portal.

Please give me some advices,

Thanks