Hi Zhipeng.  The xml file is attached as a txt file.

Please change the file extension from XML to TXT before uploading.

Hi Mike. I've noticed that you are using a variable value in the task. I created a simplified process like below (also attached), in which the workflow task did nothing but delay one minute.  It works fine for me: the value of variable '/Submitter_ID' in 'User Task' is the submitter of task 'Approval' and no instance suspends.

So I am wondering that whether the Workflow task changed the variable value?

The workflow task does not change the /Submitter_ID variable.  It retrieves variables from the Forms process and passes those values to a SQL query, which returns a Response value back to the Forms process.  The task that follows the workflow, AP Clerk, is set up to go to the /Submitter_ID, which hopefully is the person who just submitted the form and not the original submitter that started the Forms process. 

I have noticed that you are using the variable in multiple forms. Because the default value is only filled in when the variable has no value, this field would not be updated when used in multiple forms.

You can try to use process variable instead of field variable. And I tried to use '/_initiator' which works fine in my process.