What are the steps to reproduce your problem?
Create a bpmn model which has a user task, by not defining any user interface the generic task form will appear in task list when the process is run.
On this task form a Business Key field is visible

What is the problem?
The business key field is neither populated with the defined business key for the process instance or affected by any input that is added to it. There is no reason for this field to show up

What would be the expected behavior:
That the business key field only appear on start forms not on task forms.

Hints (optional):
I've attached a bpmn model to view it