How to only update subtask's field values?

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Hi, 

At one point of my workflow transition,

it shows a pop-up screen to fill in field A and B

and post function automatically creates a subtask.

  

However, I noticed that when a user fills in field A and B values different from its parent issue's - it syncs field A and B values in parent and subtask issues. 

How do I leave parent issue's fields and only change subtask's field values that user provided to the pop-up screen?

I tried Fields to copy>Custom: every field except field A and B

BUT it will update parent issue with values entered into the pop-up screen.

and the subtask wouldn't have field A and B at all.

Thank you.